Remove dead code
The configure check for HAVE_DECL_LLVMORCREGISTERPERF was removed by
e9a9843e138, but some code guarded by it was left. (That commit
removed the "register" calls but left the "unregister" calls.) That
code cannot be reached anymore, so remove it.
Reported-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://www.postgresql.org/message-id/
5539b16c-cff7-46d5-9621-
c3fb6b549e9e@iki.fi