[flang] Remove test from #69121 to fix gcc build with gcc < 10.0

ISO_Fortran_binding.h was only added to in gcc 10.0. Flang should be
buildable with older versions. Remove the test until a safe way to
check that the compiler can run the test (that it is clang from the
build for instance).

Fix bots failure https://lab.llvm.org/buildbot/#/builders/181/builds/24526
Also in:

https://lab.llvm.org/buildbot/#/builders/160
https://lab.llvm.org/buildbot/#/builders/268
https://lab.llvm.org/buildbot/#/builders/181