webadmin,userportal: Drop support for IE6-7

This patch removes "ie6" user agent from default GWT compilation,
which also means less permutations to compile and faster builds
in general.

Change-Id: I5820bfe4147d5429714ed6388bcc8d62ecb4e40c
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>