Revert "Partially restore hack"

This reverts commit 303fad8.

lib/bundler/version.rb

@@ -3,24 +3,6 @@
 module Bundler
   VERSION = "2.1.0.pre.1".freeze
 
-  if ENV["BUNDLE_SPEC_RUN"]
-    # We're doing this because we might write tests that deal
-    # with other versions of bundler and we are unsure how to
-    # handle this better.
-    def self.overwrite_loaded_gem_version
-      begin
-        require "rubygems"
-      rescue LoadError
-        return
-      end
-      return unless bundler_spec = Gem.loaded_specs["bundler"]
-      return if bundler_spec.version == VERSION
-      bundler_spec.version = Bundler::VERSION
-    end
-    private_class_method :overwrite_loaded_gem_version
-    overwrite_loaded_gem_version
-  end
-
   def self.bundler_major_version
     @bundler_major_version ||= VERSION.split(".").first.to_i
   end