Revert "frontend: Add custom field serializer for RepoImage"

This reverts commit 30a941a4b3ed8e46053880a5fd6de325907a9b52, as it was
a temporary solution for GWT RPC serialization issues that have been now
solved in other patches.

Change-Id: I1619f4b1d3b651cdb1a6173d536e5b8ab01e66ca
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>