Remove all version overriding stuff

By doing this we avoid circular requires (`rubygems` requires `bundler` via
`USE_GEMDEPS`, `bundler` requires `rubygems` when loading its own version), and
also redefinition warnings when the `bundler.gemspec` is evaluated with
`rubygems` already loaded (for example, when running `ruby setup.rb`
from `rubygems` repo).

We seem to already have other mechanisms in place for overriding the
loaded bundler version (`BUNDLER_SPEC_VERSION`), so this seems
unnecessary? Let's see what CI says.

lib/bundler/version.rb

@@ -1,23 +1,7 @@
 # frozen_string_literal: false
 
 module Bundler
-  # 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.
-  VERSION = "2.1.0.pre.1".freeze unless defined?(::Bundler::VERSION)
-
-  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
+  VERSION = "2.1.0.pre.1".freeze
 
   def self.bundler_major_version
     @bundler_major_version ||= VERSION.split(".").first.to_i