lib: remove unused frr_pthread_yield()

lib: remove unused frr_pthread_yield()

OS-level yield is generally a bad and possibly dangerous idea.  If the
thread should be suspended, there should always be something to wait on,
or it turns into busy waiting.  And if it's "just giving something else
the chance to run" - that's the kernel's job to determine, and the
kernel will do so while considering priorities, cgroups, and whatnot.
Let it do its job.

Signed-off-by: David Lamparter <equinox@diac24.net>