IntelLLVM: Revert accidental use of -external:I with Fortran compilers

In commit 8218aed118 (IntelLLVM: support marking include paths as SYSTEM
directories, 2023-08-15, v3.29.0-rc1~81^2) this flag was added for the C
and C++ compilers, but was accidentally added for Fortran too.  Remove
it for the latter, as it is unsupported.

Issue: #25807
1 file changed