Delete multiple rows from a grid.

I have made some changes in your code. Let me know if this is what you wanted. 

a!localVariables(
  local!selection,
  {
    a!gridLayout(
      headerCells: {
        a!gridLayoutHeaderCell(
          label: "Precio de Colocación",
          align: "CENTER"
        ),
        a!gridLayoutHeaderCell(
          label: "Número de TÃ%ADtulos",
          align: "CENTER"
        ),
        a!gridLayoutHeaderCell(label: "Importe", align: "CENTER"),
        a!gridLayoutHeaderCell(label: "Casa de Bolsa", align: "CENTER"),
        a!gridLayoutHeaderCell(
          label: "Tipo de Registro",
          align: "CENTER"
        )
      },
      columnConfigs: {
        a!gridLayoutColumnConfig(weight: 2),
        a!gridLayoutColumnConfig(weight: 2),
        a!gridLayoutColumnConfig(weight: 2),
        a!gridLayoutColumnConfig(weight: 1),
        a!gridLayoutColumnConfig(weight: 1)
      },
      rows: {
        a!forEach(
          items: ri!colocacion,
          expression: {
            a!gridRowLayout(
              id: fv!index,
              contents: {
                a!floatingPointField(
                  align: "CENTER",
                  value: index(fv!item, "precioColocacion", null),
                  saveInto: fv!item.precioColocacion
                ),
                a!floatingPointField(
                  align: "CENTER",
                  value: index(fv!item, "numeroTitulos", null),
                  saveInto: fv!item.numeroTitulos
                ),
                a!floatingPointField(
                  align: "CENTER",
                  value: index(fv!item, "importe", null),
                  saveInto: fv!item.importe
                ),
                a!dropdownField(
                  value: index(fv!item, "casaBolsa", null),
                  saveInto: fv!item.casaBolsa,
                  choiceLabels: "ACTIN",
                  choiceValues: "ACTIN",
                  placeholder: "Seleccione"
                ),
                a!dropdownField(
                  value: index(fv!item, "tipoRegistro", null),
                  choiceLabels: "ACTIN",
                  choiceValues: "ACTIN",
                  placeholder: "Seleccione",
                  saveInto: fv!item.tipoRegistro,
                  
                )
              }
            )
          }
        ),
        a!gridRowLayout(
          selectionDisabled: true,
          id: 9999,
          showWhen: if(
            a!isNotNullOrEmpty(ri!colocacion),
            true,
            false
          ),
          contents: {
            a!richTextDisplayField(),
            a!richTextDisplayField(
              align: "CENTER",
              value: if(
                isnull(index(ri!colocacion, "importe", null)),
                null,
                sum(ri!colocacion.numeroTitulos)
              )
            ),
            a!richTextDisplayField(),
            a!richTextDisplayField(),
            a!richTextDisplayField(),
            
          }
        )
      },
      selectable: true,
      selectionValue: local!selection,
      selectionSaveInto: local!selection
    ),
    a!buttonArrayLayout(
      align: "CENTER",
      buttons: {
        a!buttonWidget(
          label: "Agregar",
          submit: true,
          saveInto: {
            a!save(
              ri!colocacion,
              append(ri!colocacion, null)
            )
          },
          style: "NORMAL",
          
        ),
        a!buttonWidget(
          label: "Eliminar",
          submit: true,
          style: "DESTRUCTIVE",
          saveInto: a!save(
            ri!colocacion,
            remove(ri!colocacion, local!selection)
          )
        )
      }
    )
  }
)