build: Removal of unsed gwt maven plugin repo.

The removed repo is unused as the gwt maven plugin is 2.3.0
while currently this plugin repo points to 2.1.0.M3 plugin version,

The current gwt maven plugin is publicly available in the standard maven
repositories thus a special plugin repo is unncessary.

Change-Id: Ifa83712955757ac3dd3f0478bd352ab5b2ca9ba4
Signed-off-by: Asaf Shakarchi <asaf@redhat.com>