gl: win32: do not call SetParent in release_parent_win_id()

gl: win32: do not call SetParent in release_parent_win_id()

When called from gst_gl_window_win32_close(), internal window
could not exist, and if it does it's going to be destroyed just
after that anyway. Also it causes window_proc() to be called
and crash because it gets a NULL context.

When called from gst_gl_window_win32_set_window_handle() we are
going to set another parent anyway, and it's probably better to
reparent directly instead of passing by a NULL parent which could
cause the internal window to popup briefly.

https://bugzilla.gnome.org/show_bug.cgi?id=749601