Bug#49898 - Fix for bug#37408 introduces a linker error

the declaration of THR_LOCK_myisam_mmap in mi_static 
is redundant as it accessible via the extern declaration 
in include/myisam.h

myisam/mi_static.c:
  Bug#49898 - Fix for bug#37408 introduces a linker error
  
  Remove THR_LOCK_myisam_mmap declaration as it is redundant

myisam/mi_static.c

@@ -41,7 +41,6 @@ my_off_t myisam_max_temp_length= MAX_FILE_SIZE;
 ulong    myisam_bulk_insert_tree_size=8192*1024;
 ulong    myisam_data_pointer_size=4;
 ulonglong    myisam_mmap_size= SIZE_T_MAX, myisam_mmap_used= 0;
-pthread_mutex_t  THR_LOCK_myisam_mmap;
 
 static int always_valid(const char *filename __attribute__((unused)))
 {