[mlir][openacc] Cleanup acc.exit_data from old data clause operands

Since the new data operand operations have been added in D148389 and
adopted on acc.exit_data in D149601, the old clause operands are no longer
needed.

The LegalizeDataOpForLLVMTranslation will become obsolete when all
operations will be cleaned. For the time being only the appropriate
part are being removed.

processOperands will also receive some updates once all the operands
will be coming from an acc data operand operation.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D150145