Remove unused functions from isc_thread API

The isc_thread_setaffinity call was removed in !5265 (merged) and we are not going to restore it because it was proven that the performance is better without it. Additionally, remove the already disabled cpu system test.

The isc_thread_setconcurrency function is unused and also calling pthread_setconcurrency() on Linux has no meaning, formerly it was added because of Solaris in 2001 and it was removed when taskmgr was refactored to run on top of netmgr in !4918 (merged).

Closes #3130 (closed)