[Flang][MLIR][OpenMP] Remove the early outlining interface (#78450)

After the removal of the OpenMP early outlining MLIR pass in #67319, the
`EarlyOutliningInterface` stopped doing any useful work. It used to be
necessary to tie the name of the function from which a target region was
outlined to that new function, so it would be used when translating to
LLVM IR in place of the outlined function's name.

This is not necessary anymore, so this patch removes all references to
this interface and uses of the `omp.outline_parent_name` discardable
attribute in tests.