Remove second declaration of jl_n_threads

The declaration has been moved to julia.h, which is already included
in this file.

(cherry picked from commit cb2d476)

Author: imciner2

src/threading.h

@@ -13,7 +13,6 @@ extern "C" {
 #define PROFILE_JL_THREADING            0
 
 extern jl_ptls_t *jl_all_tls_states JL_GLOBALLY_ROOTED; /* thread local storage */
-extern JL_DLLEXPORT int jl_n_threads;   /* # threads we're actually using */
 
 typedef struct _jl_threadarg_t {
     int16_t tid;