[gdb/testsuite] Remove inferior output in gdb.base/foll-vfork.exp

Test-case gdb.base/foll-vfork.exp has inferior output that is not needed, but
which makes the regexp matching more difficult (see commit 1f28b70d
"[gdb/testsuite] Fix regexp in gdb.base/foll-vfork.exp").

Remove the inferior output, and revert commit 1f28b70d to make the matching
more restrictive.

Tested on x86_64-linux.