no need to check whether mq_master is nil in the GC work-stealing loop (#53899)

This is not needed after #53355.

Author: d-netto

src/gc.c

@@ -2976,11 +2976,10 @@ JL_EXTENSION NOINLINE void gc_mark_loop_serial(jl_ptls_t ptls)
 
 void gc_mark_and_steal(jl_ptls_t ptls)
 {
-    jl_gc_markqueue_t *mq = &ptls->mark_queue;
-    jl_gc_markqueue_t *mq_master = NULL;
     int master_tid = jl_atomic_load(&gc_master_tid);
     assert(master_tid != -1);
-    mq_master = &gc_all_tls_states[master_tid]->mark_queue;
+    jl_gc_markqueue_t *mq = &ptls->mark_queue;
+    jl_gc_markqueue_t *mq_master = &gc_all_tls_states[master_tid]->mark_queue;
     void *new_obj;
     jl_gc_chunk_t c;
     pop : {
@@ -3024,12 +3023,10 @@ void gc_mark_and_steal(jl_ptls_t ptls)
             }
         }
         // Try to steal chunk from master thread
-        if (mq_master != NULL) {
-            c = gc_chunkqueue_steal_from(mq_master);
-            if (c.cid != GC_empty_chunk) {
-                gc_mark_chunk(ptls, mq, &c);
-                goto pop;
-            }
+        c = gc_chunkqueue_steal_from(mq_master);
+        if (c.cid != GC_empty_chunk) {
+            gc_mark_chunk(ptls, mq, &c);
+            goto pop;
         }
         // Try to steal pointer from random GC thread
         for (int i = 0; i < 4 * jl_n_markthreads; i++) {
@@ -3047,11 +3044,9 @@ void gc_mark_and_steal(jl_ptls_t ptls)
                 goto mark;
         }
         // Try to steal pointer from master thread
-        if (mq_master != NULL) {
-            new_obj = gc_ptr_queue_steal_from(mq_master);
-            if (new_obj != NULL)
-                goto mark;
-        }
+        new_obj = gc_ptr_queue_steal_from(mq_master);
+        if (new_obj != NULL)
+            goto mark;
     }
 }