BUILD FIX (r142576): WK1 build fails when ENABLE(DELETION_UI) is off
<https://bugs.webkit.org/show_bug.cgi?id=109534>

Fixes the following build failure:

    WebEditorClient.mm:243:23: error: out-of-line definition of 'shouldShowDeleteInterface' does not match any declaration in 'WebEditorClient'
    bool WebEditorClient::shouldShowDeleteInterface(HTMLElement* element)
                          ^~~~~~~~~~~~~~~~~~~~~~~~~

* WebCoreSupport/WebEditorClient.mm:
(WebEditorClient::shouldShowDeleteInterface): Protect with
ENABLE(DELETION_UI) macro.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142629 268f45cc-cd09-0410-ab3c-d52691b4dbfc