Remove GTK2 code

GTK2 code was historically kept around to keep the electron build working.  But
as of Nov 26, 2017, electron now uses GTK3 [1], so it should now be safe to
remove all of the old GTK2-only code.

[1] https://github.com/electron/electron/issues/2927#issuecomment-347092515

BUG=876558
R=sky

Change-Id: I78402053ae508ccd8fc7ac73697a861bb1ebacf6
Reviewed-on: https://chromium-review.googlesource.com/1183858
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585265}

docs/linux_debugging_gtk.md

@@ -8,7 +8,7 @@ for notes on how to make GTK warnings fatal.
 
 ## Using GTK Debug packages
 
-    sudo apt-get install libgtk2.0-0-dbg
+    sudo apt-get install libgtk-3-0-dbg
 
 Make sure that you're building a binary that matches your architecture (e.g.
 64-bit on a 64-bit machine), and there you go.
@@ -21,7 +21,7 @@ doing:
 
 ```shell
 $ cd /my/dir
-$ apt-get source libgtk2.0-0
+$ apt-get source libgtk-3-0
 $ gdb ...
 (gdb) set substitute-path /build/buildd /my/dir
 ```
@@ -46,14 +46,6 @@ And then run Chrome with
 
     GTK_MODULES=gtkparasite ./out/Debug/chrome
 
-### ghardy
-
-If you're within the Google network on ghardy, which is too old to include
-gtkparasite, you can do:
-
-    scp bunny.sfo:/usr/lib/gtk-2.0/modules/libgtkparasite.so /tmp
-    sudo cp /tmp/libgtkparasite.so /usr/lib/gtk-2.0/modules/libgtkparasite.so
-
 ## GDK_DEBUG
 
 Use `GDK_DEBUG=nograbs` to run GTK+ without grabs. This is useful for gdb