Remove Flow's mw-ui-button overrides

The "beveled bottom" Agora CSS is now in mediawiki.ui.button in core.

We still need agora-override-forms.less for the left blue bar on input
fields until bug 62281 is addressed in core, and the current content
of agora-override-buttons.less is needed for button size overrides.

Bug: 62297
Change-Id: I8dd6b8304dc8941ef4f27566375c77269eafb05f