Remove fastpath.c's lame attempt at caching function lookup info across

Remove fastpath.c's lame attempt at caching function lookup info across
calls.  This has never actually cached anything, because postgres.c does
each fastpath call as a separate transaction command, and so fastpath.c
would always decide that its cache was outdated.  If it had worked, it
would now be failing for calls of oldstyle functions due to dangling
pointers in the FmgrInfo struct.  Rip it out for simplicity and bug-
proofing.