Remove explicit dependency on bouncy castle

Bouncy Castle was added explicitly as a test dependency. This caused
the tests to fail, because a version gap between Gerrit core and the
plugin happened.

Since the Gerrit plugin API does also provide Bouncy Castle, it is not
required to add it explicitly.

Change-Id: Ia1c97b143bee6912ea0d4211ee20d130e2ffad8c