Remove unused private method

The private method is_module_package? was only ever referenced by code
that was removed in 2014 by 3f05274. Because this method is both private
and not referenced anywhere else in the code base, this commit removes
the is_module_package? method.

Author: mhashizume

lib/puppet/module_tool/applications/installer.rb

@@ -395,18 +395,6 @@ def resolve_install_conflicts(graph, is_dependency = false)
           end
         end
       end
-
-      #
-      # Check if a file is a vaild module package.
-      # ---
-      # FIXME: Checking for a valid module package should be more robust and
-      # use the actual metadata contained in the package. 03132012 - Hightower
-      # +++
-      #
-      def is_module_package?(name)
-        filename = File.expand_path(name)
-        filename =~ /.tar.gz$/
-      end
     end
   end
 end