Revert "[Flang] Add partial support for lowering procedure pointer assignment. (#70461)"

This reverts commit e07fec10.

This change appears to have broken following buildbots:
https://lab.llvm.org/buildbot/#/builders/176
https://lab.llvm.org/buildbot/#/builders/179
https://lab.llvm.org/buildbot/#/builders/184
https://lab.llvm.org/buildbot/#/builders/197
https://lab.llvm.org/buildbot/#/builders/198

All bots fails in testsuite where following tests seems broken:
(eg: https://lab.llvm.org/buildbot/#/builders/176/builds/7131)

test-suite::gfortran-regression-compile-regression__proc_ptr_46_f90.test
test-suite::gfortran-regression-compile-regression__proc_ptr_37_f90.test