Clean up the framework postprocessing phase

Clean up the framework postprocessing phase
https://bugs.webkit.org/show_bug.cgi?id=141544

Reviewed by Dan Bernstein.

Rename WK_API_AVAILABILITY_ENABLED to WK_FRAMEWORK_HEADER_POSTPROCESSING_ENABLED to better match what
it is we're doing. Also, rename the associated script. Remove a couple of now unnecessary #defines, and
change everything we can to be feature-based instead of OS based.

* Configurations/WebKit.xcconfig:
* Shared/API/Cocoa/WKFoundation.h:
* WebKit2.xcodeproj/project.pbxproj:
* mac/postprocess-framework-headers.sh: Renamed from Source/WebKit2/mac/rewrite-availability-macros.sh.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@180052 268f45cc-cd09-0410-ab3c-d52691b4dbfc