Remove build files from `make clean`
Remove build files from `make clean`

Those don't exist anymore. We build directly to `release` (and only the
one UMD build now).

theWebalyst/remotestorage.js,remotestorage/remotestorage.js,theWebalyst/remotestorage.js,remotestorage/remotestorage.js,remotestorage/remotestorage.js