Stop linking pthread.

It was only needed by gtest, which is no longer used.
(Intesting note: I checked when the -lpthread flag was added, and it's
been around since the first revision of build.ninja, which used to be
checked in before configure.py existed. Back then, it looks like '@' was
used to dereference built-in variables, and build outputs were also
prefixed by '@'!).
(Note: All the perf tests only linked to `libs`, while `ninja_test` linked to `test_libs`. But since this line here didn't make a copy, `test_libs` and `libs` referred to the same list, and both perf tests and unit tests ended up linking in pthreads. This looks like it was unintentional.)