CLEANUP: fd: remove the now unneeded fd_mig_lock

This is not needed anymore since we don't use it when setting the running
mask anymore.
diff --git a/include/haproxy/fd.h b/include/haproxy/fd.h
index 5e9a967..4160252 100644
--- a/include/haproxy/fd.h
+++ b/include/haproxy/fd.h
@@ -67,11 +67,6 @@
  */
 int fd_takeover(int fd, void *expected_owner);
 
-/* lock used by FD migration */
-#ifndef HA_HAVE_CAS_DW
-__decl_thread(extern HA_RWLOCK_T fd_mig_lock);
-#endif
-
 ssize_t fd_write_frag_line(int fd, size_t maxlen, const struct ist pfx[], size_t npfx, const struct ist msg[], size_t nmsg, int nl);
 
 /* close all FDs starting from <start> */
diff --git a/src/fd.c b/src/fd.c
index 138a8fd..f746dff 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -328,10 +328,6 @@
 	_HA_ATOMIC_DEC(&ha_used_fds);
 }
 
-#ifndef HA_HAVE_CAS_DW
-__decl_thread(__decl_rwlock(fd_mig_lock));
-#endif
-
 /* Deletes an FD from the fdsets. The file descriptor is also closed, possibly
  * asynchronously. Only the owning thread may do this.
  */
@@ -354,13 +350,7 @@
 	 */
 
 	HA_ATOMIC_OR(&fdtab[fd].running_mask, tid_bit);
-#ifndef HA_HAVE_CAS_DW
-	HA_RWLOCK_WRLOCK(OTHER_LOCK, &fd_mig_lock);
-#endif
 	HA_ATOMIC_STORE(&fdtab[fd].thread_mask, 0);
-#ifndef HA_HAVE_CAS_DW
-	HA_RWLOCK_WRUNLOCK(OTHER_LOCK, &fd_mig_lock);
-#endif
 	if (fd_clr_running(fd) == 0)
 		_fd_delete_orphan(fd);
 }