drop grunt support

We have a Gruntfile for doing some CSS linting checks, which was added
back in 7a2ff7a (Gruntfile and package.json, 2015-03-23). This is our
only use of nodejs/npm, so it also brings in a package.json file. Some
of the packages we use are out of date and no longer supported (e.g.,
grunt-autoprefixer is deprecated in favor of grunt-postcss). This causes
dependabot to complain quite a bit, but there is no easy solution (e.g.,
"npm audit fix" chokes because the ancient autoprefixer wants a
similarly ancient version of grunt).

Given that I have never used these, and that we don't even run them as
part of CI, my feeling is that periodically dealing with npm package
updates is not worth it. If there were active CSS work going on in the
repo (which was happening when they were added), they might be carrying
their weight, but that's not really the case these days.

So this commit removes them entirely, making the problem go away.

Author: peff