Replace all uses of \providecommand with \newcommand.

Replace all uses of \providecommand with \newcommand.

We generally want to know if some of our macros aren't being used. But
before introducing the \ifx guards on file includes, we were forced to
use \providecommand in e.g. mjo-common.tex, since it gets included
more than once. Now that we have the guards, change all
\providecommands to \newcommands.