py/mpstate: Don't declare mp_thread_get_state().

It's instead declared by the include of `py/mpthread.h`.

Signed-off-by: Damien George <damien@micropython.org>

py/mpstate.h

@@ -310,7 +310,6 @@ extern mp_state_ctx_t mp_state_ctx;
 #define MP_STATE_MAIN_THREAD(x) (mp_state_ctx.thread.x)
 
 #if MICROPY_PY_THREAD
-extern mp_state_thread_t *mp_thread_get_state(void);
 #define MP_STATE_THREAD(x) (mp_thread_get_state()->x)
 #define mp_thread_is_main_thread() (mp_thread_get_state() == &mp_state_ctx.thread)
 #else