[MLIR][OpenMP] remove now unnecessary getUsedValuesDefinedAbove call from convertTargetOp (#72904)

This block of code was here to create pseudo handling of implicit
captures in target regions to prevent gfortran test regressions and
allow certain pieces of code to function, however, with the introduction
of the IFA patch which adds proper handling of implicits by adding them
to the map operands list alongside explicit mappings at the initial
Fortran -> MLIR generation phase this should no longer be required and
may cause some adverse affects at worse in the future.