webadmin,userportal: Fix compile error when using gwtdev profile

Trying to debug WebAdmin/UserPortal via GWT Hosted Mode:

$ mvn gwt:debug -Pgwt-admin,gwt-user,gwtdev -Dgwt.noserver=true

results in Java compilation error, since GwtCachingFilter
calls java.text.DateFormat methods that are currently not
emulated via custom java.text.DateFormat GWT override.

This patch adds no-op (empty) implementations of required
methods to fix this error.

Note that GWT overrides affect GWT Hosted Mode only;
debugging server-side module (e.g. GwtCachingFilter in
'frontend' module) as Remote Java Application attaches
to server-side JVM that is unaware of GWT overrides.

Change-Id: Ic0cc662037f13ca8c19c565ebb082b8b8b82c7bc
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>