Makefile comment: remove reference to tools/thread/thread_test

You can't compile thread_test alone anymore, and the location moved too.

Reported-by: Tom Lane

Discussion: https://postgr.es/m/1062278.1603819969@sss.pgh.pa.us

Backpatch-through: 9.5

src/template/netbsd

 # src/template/netbsd
-# tools/thread/thread_test must be run
 
 # Extra CFLAGS for code that will go into a shared library
 CFLAGS_SL="-fPIC -DPIC"