tests/header_guard.sh: Drop check against Freeciv.h
There is no longer Freeciv.h. Check against it in header_guard.sh is obsolete. Clean it out.
Freeciv.h was generated as part of gtk2-client build, so targeting this to branches where gtk2-client has been dropped.
There is no longer Freeciv.h. Check against it in header_guard.sh is obsolete. Clean it out.