Remove 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).

But most importantly, we avoid leaking from a bundler installation to a
different one, something quite common since bundler is a default gem.
The previous hack meant acting like that didn't happen, but seems
actually quite dangerous.

We can do this due to the previous work of making all of bundler
internal requires not rely on the LOAD_PATH, and thus making it
impossible to leak to another copy of bundler.

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