[GTK] Stop trying to install WebKit1 documentation
https://bugs.webkit.org/show_bug.cgi?id=131278

Reviewed by Martin Robinson.

* Source/PlatformGTK.cmake: Remove install() call since the WebKit1 documentation is not
built anymore since r166584. This fixes `make install'.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@166850 268f45cc-cd09-0410-ab3c-d52691b4dbfc