gdb/darwin: remove not-so-harmless spurious call to wait4

As seen in https://sourceware.org/bugzilla/show_bug.cgi?id=24069 this
code will typically wait4() a second time on the same process that was
already wait4()'d a few lines above. While this used to be
harmless/idempotent (when we assumed that the process already exited),
this now causes a deadlock in the WIFSTOPPED case.

The early (~2019) history of bug #24069 cautiously suggests to use
WNOHANG instead of outright deleting the call. However, tests on the
current version of Darwin (Big Sur) demonstrate that gdb runs just fine
without a redundant call to wait4(), as would be expected.
Notwithstanding the debatable value of conserving bug compatibility with
an OS release that is more than a decade old, there is scant evidence of
what that double-wait4() was supposed to achieve in the first place - A
cursory investigation with `git blame` pinpoints commits bb00b29d78
and a80b95ba67 from the 2008-2009 era, but fails to answer the
"why" question conclusively.

Co-Authored-By: Philippe Blain <levraiphilippeblain@gmail.com>
Change-Id: Id4e4415d66d6ff6b3552b60d761693f17015e4a0

gdb/darwin-nat.c

@@ -1113,9 +1113,6 @@ darwin_nat_target::decode_message (mach_msg_header_t *hdr,
 	      inferior_debug (4, _("darwin_wait: pid=%d exit, status=0x%x\n"),
 			      res_pid, wstatus);
 
-	      /* Looks necessary on Leopard and harmless...  */
-	      wait4 (inf->pid, &wstatus, 0, NULL);
-
 	      return ptid_t (inf->pid);
 	    }
 	  else