[flang][driver] Remove `flang -help` tests (#89504)

These tests were introduced when the Flang driver was originally
incepted. Back then, `flang-new` only supported a handful of basic
options (e.g. `-v`, `-help`, `-E`) and those `-help` tests were used to
guard against Clang options "leaking" into `flang-new -help`.

With the introduction of the concept of "visibility" (see
https://reviews.llvm.org/D157837 and https://reviews.llvm.org/D157151),
these tests are effectively redundant. Also, with the number of flags
growing, these tests are getting rather brittle.