theme: Unbind GtkTextView's Ctrl-Shift-Delete accelerator.

This is needed so the combination can be used to open the
"Clear Browsing Data" dialog while the omnibox has the
focus.

Change-Id: I07c23d9636941a82116297893c456d4937e9c20a

BUG=chromium-os:8986
TEST=installed it in conjunction with http://codereview.chromium.org/5958007/ and checked that the shortcut worked

Review URL: http://codereview.chromium.org/6086008