[flang] Remove `%f18` from LIT configuration files

`%f18` was originally introduced to represent the old Flang driver,
`f18`. With the introduction of the new driver, `flang-new`, we have
been switching to `%flang` (compiler driver) and `%flang_fc1` (frontend
driver) as more generic alternatives.

As most tests have been portend to use the new LIT variables instead of
`%f18`, this is good time to remove it from lit.cfg.py. There's only one
test left that requires the old driver to run. It's updated with:
```
! REQUIRES: old-flang-driver
```
This way we preserve its semantics while reducing the number of
variables in LIT configuration.

Differential Revision: https://reviews.llvm.org/D101281