Simplify redundant fallbacks

Fix #261

fix \ProvidesPackage{} for highcontrast theme

Format table rules following booktabs advice

Removes warning:

LaTeX Warning: You have requested package `beamercolorthememetropolis-highcontrast',
               but the package provides `beamercolorthememetropolis-minimal'.

Simplify a patch

This avoids looking for an internal command that will be removed
in a future update.