removing --dialog-element from build-webkit --help option message

removing --dialog-element from build-webkit --help option message
https://bugs.webkit.org/show_bug.cgi?id=120955

Patch by Santosh Mahto <santosh.ma@samsung.com> on 2013-09-07
Reviewed by Daniel Bates.

--dialog-element option is useless while building as
Dialog Element support has been removed in following changeset.
http://trac.webkit.org/changeset/154835
http://trac.webkit.org/changeset/154870

* Scripts/webkitperl/FeatureList.pm:
Removed --dialog-element option message.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@155256 268f45cc-cd09-0410-ab3c-d52691b4dbfc