Merge #7398

7398: Remove unnecessary lines from `bundler/setup` r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

These lines in the `bundler/setup` file are not necessary.

### What was your diagnosis of the problem?

Bundler no longer needs to be in the `$LOAD_PATH` once a version of itself has been loaded.

In any case, the `Metadata` source takes care of adding `bundler` to the index:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/source/metadata.rb#L13-L30

and then that spec is added to the resolution here:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/definition.rb#L180-L184

And from the resulting set of specs, the `$LOAD_PATH` is setup:

https://github.com/bundler/bundler/blob/e70643c1be3a4417bd537d7e63470265465e693e/lib/bundler/runtime.rb#L25-L38

 So `bundler` will be present in the `$LOAD_PATH` anyways, and the lines being removed here will never be useful.

### What is your fix for the problem, implemented in this PR?

My fix is to remove these lines to make the code easier to follow.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>
(cherry picked from commit bf7df42)

lib/bundler/setup.rb

@@ -19,8 +19,4 @@
   else
     Bundler.ui.silence { Bundler.setup }
   end
-
-  # Add bundler to the load path after disabling system gems
-  bundler_lib = File.expand_path("../..", __FILE__)
-  $LOAD_PATH.unshift(bundler_lib) unless $LOAD_PATH.include?(bundler_lib)
 end