[GTK] No need to manually add system paths to jhbuildrc
https://bugs.webkit.org/show_bug.cgi?id=110850

Reviewed by Gustavo Noronha Silva.

* gtk/jhbuildrc: Remove calls to addpath() for items that jhbuild
already takes care of.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144628 268f45cc-cd09-0410-ab3c-d52691b4dbfc