Remove BUILDING_ON_LEOPARD now that no ports build on Leopard

Remove BUILDING_ON_LEOPARD now that no ports build on Leopard
https://bugs.webkit.org/show_bug.cgi?id=90249

Reviewed by Ryosuke Niwa.

Source/WebCore:

I don't think I quite got it all yet, but this is another step towards
removing Leopard support in WebCore.

* WebCore.exp.in:
* page/AlternativeTextClient.h:
* page/ContextMenuController.cpp:
(WebCore::ContextMenuController::populate):
* platform/graphics/cg/ImageBufferDataCG.h:
* platform/graphics/mac/GraphicsContextMac.mm:
(WebCore::GraphicsContext::drawLineForDocumentMarker):
* platform/graphics/mac/MediaPlayerPrivateQTKit.mm:
(WebCore::MediaPlayerPrivateQTKit::setClosedCaptionsVisible):
* platform/mac/WebCoreSystemInterface.h:
* platform/network/Credential.h:
* platform/network/cf/ResourceRequestCFNet.cpp:
(WebCore):
(WebCore::initializeMaximumHTTPConnectionCountPerHost):
* platform/text/TextChecking.h:
(WebCore):
* rendering/RenderLayerBacking.cpp:
(WebCore::RenderLayerBacking::containsPaintedContent):

Source/WebKit2:

* WebProcess/Plugins/Netscape/NetscapeBrowserFuncs.cpp:
(WebKit::NPN_GetValue):
* WebProcess/WebCoreSupport/WebEditorClient.cpp:
(WebKit::WebEditorClient::shouldEraseMarkersAfterChangeSelection):
* WebProcess/WebCoreSupport/WebEditorClient.h:

Source/WTF:

* wtf/ThreadingPthreads.cpp:
(WTF::initializeCurrentThreadInternal):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121572 268f45cc-cd09-0410-ab3c-d52691b4dbfc