Revert "Making sure mediawiki.ui buttons use regular box-sizing"

Maryana points out that this breaks the login button on the signup form
As you point out mw-ui-button is already border-box by default.
This reverts commit d84ba2ae39163fa820013016510832b48fc1d088.

Change-Id: I0412c476d1bc46d844d8f229a81f4821027b8338