BUILD: fix build without thread

Cyril Bonté reported that commit f9cc07c25b broke the build without
thread.

We don't need to initialise tid = 0 in mworker_loop, so we could
completely remove it.
diff --git a/src/haproxy.c b/src/haproxy.c
index d596356..3bf1f3b 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -867,8 +867,6 @@
 	mworker_catch_sigchld(NULL); /* ensure we clean the children in case
 				     some SIGCHLD were lost */
 
-	tid = 0;
-
 	global.nbthread = 1;
 	relative_pid = 1;
 	pid_bit = 1;