Make NO_PTHREADS the sole thread configuration variable

Make NO_PTHREADS the sole thread configuration variable

When the first piece of threaded code was introduced in commit 8ecce684, it
came with its own THREADED_DELTA_SEARCH Makefile option. Since this time,
more threaded code has come into the codebase and a NO_PTHREADS option has
also been added. Get rid of the original option as the newer, more generic
option covers everything we need.

Signed-off-by: Dan McGee <redacted>
Signed-off-by: Junio C Hamano <redacted>