meson: Remove old todo comment about pthread stubs.

Noticed while making my last change.  pthread stubs are long dead, we
actually use threads all over now.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7695>

meson.build

@@ -1657,8 +1657,6 @@ else
   asan_c_args = ['-DBUILT_WITH_ASAN=0']
 endif
 
-# pthread stubs. Lets not and say we didn't
-
 if host_machine.system() == 'windows'
   # Prefer the winflexbison versions, they're much easier to install and have
   # better windows support.