remove build directory
remove build directory

BREAKING CHANGE: the build directory is no longer included in the
distributed package. build/stats.json can be found at
dist/stats/primer.json, and `require('@primer/css/build')` references
should be rewritten to `require('@primer/css/dist/primer')`.

primer/primer-css,primer/primer,primer/primer-css,primer/primer,primer/primer-css,primer/primer