Don't verify NPM cache to speed up building Opencast

Quite a while ago, we encountered various problems building the module admin-ui that required manual deletion of the NPM cache.
Later, NPM was upgraded the a newer version that solved that problems.

As a precaution, NPM cache verification was built into the build system.

Since no more problems have appeared, it should be safe to remove that precaution which shortens the build process a little (approx. 1 minute).