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

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().