[Build] Eliminate unused package version metadata feature.

 - This feature is only partially implemented, and isn't actually used by
   anything. Unfortunately, it also conflicts with some of the work I need to do
   on SE-0135 in a way that is non-trivial to fix right now. I'm ripping it out
   for now, we can land it again once this feature has been fully designed and
   implemented.

Author: ddunbar

Sources/Build/PackageVersion.swift

@@ -14,7 +14,6 @@ import XCTest
 public func allTests() -> [XCTestCaseEntry] {
     return [
         testCase(DescribeTests.allTests),
-        testCase(PackageVersionDataTests.allTests),
     ]
 }
 #endif