Remove unused OmniboxView::OnPerformDrop and related code.

These functions are not currently used nor necessary.

BUG=131660
TEST=No omnibox or toolbar drag-and-drop regressions.
R=pkasting@chromium.org

Review URL: https://codereview.chromium.org/116573009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241433 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241433