Remove a spurious target_terminal::ours() from windows_nat_target::wait()

This causes the inferior to stop with SIGTTIN if it tries to read from the
terminal after it has been continued.

See https://cygwin.com/ml/cygwin/2016-09/msg00285.html for reproduction.

Since MinGW doesn't have a tcsetpgrp(), I don't think this problem would be
observed there, but Cygwin does so target_terminal::ours() will call it.

Calling target_terminal::ours() here seems to be is no longer appropriate
after the "Merge async and sync code paths" changes (as the inferior is now
in a separate process group even in sync mode(?), which is always used on
Windows targets)

This call was added in commit c44537cf (and see
https://sourceware.org/ml/gdb-patches/2007-02/msg00167.html for what it
fixed, which is not regressed by this change)

When windows_nat_target::wait() is entered, the inferior is running (either
it's been just been started or attached to, or windows_continue() was
called), so grabbing the controlling terminal away from it here seems to be
wrong, since infrun.c takes care of calling target_terminal::ours() when the
inferior stops.

gdb/ChangeLog:

2018-08-02  Jon Turney  <jon.turney@dronecode.org.uk>

	* windows-nat.c (windows_nat_target::wait): Remove a spurious
	target_terminal::ours().

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2018-08-02  Jon Turney  <jon.turney@dronecode.org.uk>
+
+	* windows-nat.c (windows_nat_target::wait): Remove a spurious
+	target_terminal::ours().
+
 2018-09-23  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* aarch64-linux-tdep.c (aarch64_linux_supply_sve_regset): Change type

gdb/windows-nat.c

@@ -1705,8 +1705,6 @@ windows_nat_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
 {
   int pid = -1;
 
-  target_terminal::ours ();
-
   /* We loop when we get a non-standard exception rather than return
      with a SPURIOUS because resume can try and step or modify things,
      which needs a current_thread->h.  But some of these exceptions mark