Unset --offline flag

This commit undo 1cc7d84f8293b73a37ce321484aa3b09e11ed054 where I explained the reason for this pull request. In short: At least for me, 1cc7d84f8293b73a37ce321484aa3b09e11ed054 gave me no speedup, but breaks the build with clean maven. This could scare off new contributors.

Kind regards, Jentsch