<https://webkit.org/b/120051> Remove the Mail thread workaround

<https://webkit.org/b/120051> Remove the Mail thread workaround

Reviewed by Sam Weinig.

Remove the workaround for a very old version of Mail added in r39455.

* WebView/WebArchive.mm:
(-[WebArchive initWithMainResource:subresources:subframeArchives:]):
(-[WebArchive mainResource]):
(-[WebArchive subresources]):
(-[WebArchive subframeArchives]):
* WebView/WebFrameView.mm:
(-[WebFrameView initWithFrame:]):
* WebView/WebResource.mm:
(-[WebResource data]):
(-[WebResource URL]):
(-[WebResource MIMEType]):
(-[WebResource textEncodingName]):
(-[WebResource frameName]):
(-[WebResource _ignoreWhenUnarchiving]):
(-[WebResource _initWithData:URL:MIMEType:textEncodingName:frameName:response:copyData:]):
(-[WebResource _suggestedFilename]):
(-[WebResource _response]):
(-[WebResource _stringValue]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@154312 268f45cc-cd09-0410-ab3c-d52691b4dbfc