[flang] always run PolymorphicOpConversion sequentially (#90721)

It was pointed out in post commit review of
https://github.com/llvm/llvm-project/pull/90597 that the pass should
never have been run in parallel over all functions (and now other top
level operations) in the first place. The mutex used in the pass was
ineffective at preventing races since each instance of the pass would
have a different mutex.