Remove redundant dependency to commons-io

The commons-io library is a dependency of Gerrit core and Gerrit plugins
have access to Gerrit core dependencies.. Hence there is no need to add
it redundantly as a plugin dependency which had the effect that we
shade a different version of this library into the plugin jar.

Change-Id: Ifc7d5486ca9a07cc365639aec23367ae8ac7f3fe