mk/platform/Darwin.mk: Remove hard-coded SDK versions

These mappings were not really achieving anything and caused
breakage every time a new SDK was released.
Without them, pkgsrc uses the symlink to the latest SDK,
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk.
See pkgsrc-users discussion 2023-05-26 through 2023-06-09
entitled "Darwin bootstrap failure".

(bacon)