[Flang][test] Don't set PATH in flang/test/Semantics/common.sh

Currently, many `Flang :: Semantics` tests FAIL on Solaris.  The failure
mode is always the same:

  actual at 67: Name in ALLOCATE statement must be a variable name
  expect at 67: ERROR: Name in ALLOCATE statement must be a variable name

It turns out the issue is twofold here:

- The awk script embedded in `flang/test/Semantics/common.sh` uses a string
  value for `FS`, which isn't supported by traditional awk as documented in
  the GNU autoconf manual.

- Even though /usr/gnu/bin (with gawk installed as awk) is in my build
  environment's `PATH`, still /bin/awk is used

Although the proper fix would probably be to avoid the unportability, it's
easy to work around it for now.  `common.sh` above is the only script in
the whole tree that for unknown reasons overrides `PATH` to
`/usr/bin:/bin`, unnecessarily creating this mess in the first place.  Just
removing that setting easily avoids the issue.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D78550