DevTools: get rid of obsolete "Sources Panel" section in shortcuts screen

Sources Panel triggered creation of "Sources Panel" section in shortcuts
panel. The variable haven't being used anywhere in the code, and we don't
have a "Sources Panel" any more, thus its safe to remove it.

R=vsevik

Review URL: https://codereview.chromium.org/405583002

git-svn-id: svn://svn.chromium.org/blink/trunk@178430 bbb929c8-8fbe-4397-9dbb-9b2b20218538