frame: remove unnecessary server grab

meta_window_ensure_frame() creates its own grab and has a comment
claiming that it must be called under a grab too.

But the reasoning given in the comment does not seem relevant here.
We only frame non-override-redirect windows, so we are creating
the frame in response to MapRequest. There is no way that the child
could receive a MapNotify at this point, since that only happens
much later, once we go through the CALC_SHOWING queue and call
XMapWindow() from meta_window_show().

Remove the unnecessary grab.

https://bugzilla.gnome.org/show_bug.cgi?id=721345