Popups: Fix OpenPopup() being called after BeginPopup() resulting in a failure to open a popup when focus is NULL. (#4308)

Story of removed line begins in commit b80cf0a. It's purpose was to close popups when they lost focus.
Later on few other changes were introduced:
* bcc49ff - closing popups with RMB without changing window focus
* af679a1 - closing popups in FocusWindow()
These two changes covered all the cases of popup closing which made deleted line obsolete. Conveniently, now all popup closing is handled from within UpdateMouseMovingWindowEndFrame() either by calling FocusWindow() or ClosePopupsOverWindow().
2 files changed