[rubygems/rubygems] Remove code that seems unnecessary

This change was never covered with a spec, and we have recently covered
the case of partially deleted gems with specs and it works fine
(installation is "auto-healed").

https://github.com/rubygems/rubygems/commit/6e66ee4235

lib/bundler/stub_specification.rb

@@ -64,9 +64,7 @@ module Bundler
     end
 
     def full_gem_path
-      # deleted gems can have their stubs return nil, so in that case grab the
-      # expired path from the full spec
-      stub.full_gem_path || method_missing(:full_gem_path)
+      stub.full_gem_path
     end
 
     def full_require_paths