Remove standalone build mode

Since I16370e57f2 JGit is consumed as a submodule in Gerrit core. This
makes it easier to upgrade JGit in gerrit core, which means it can be
any arbitrary revision that doesn't have corresponding artifacts
available for download from a maven repository (unless they are manually
built and deployed by someone).

Change-Id: I1232264584173c9a1e2c7312d75be009c830d6ee