gdb.base/foll-fork.exp: remove gating based on target triplet

It looks like this test has some code to check at runtime the support of
fork handling of the target (see check_fork_catchpoints).  So, it seems
to me that the check based on target triplet at the beginning of the
test is not needed.  This kind of gating is generally not desirable,
because we wouldn't think of updating it when adding fork support to a
target.  For example, FreeBSD supports fork, but it wasn't listed here.

Change-Id: I6b55f2298edae6b37c3681fb8633d8ea1b5aabee