The default cache path is `vendor/cache`, not `vendor/bundle`.

This is a bug fix, so it makes no sense to make it configurable. Also,
the fix is unlikely to cause problems other than maybe needing a fresh
`bundle install` on some edge cases. So, let's ship the fix and remove
the setting.

7127: Add git and branch options to `bundle add` r=deivid-rodriguez a=tokachev

Closes #6841 These changes will allow to add a gem with the git or branch options.

```
bundle add redis --git "https://github.com/redis/redis-rb" --branch "staging"
```

The result in Gemfile will be: 
```
gem "redis", "~> 4.0", :git => "https://github.com/redis/redis-rb", :branch => "staging"
```

Co-authored-by: Baumgarten <baumgarten@localhost.localdomain>

In my opinion, it's overkill to provide a setting for how little this
setting was doing. Both types of Gemfile are supported and work
regardless of this setting. The only difference this setting would make
is the warning message one would get when having _both_ types of
Gemfiles in the same project.

I changed things so that gems.rb is always looked up first, and the
warning message in case you have both always tells you to remove Gemfile
and Gemfile.lock.

The list command was still present (just aliased to `bundle show`), even
if the setting was set to false. So the setting was at least super
confusing if not just wrong.

It also led to misleading situations such as

```
$ bundle list --help

(...)

NAME
       bundle-list - List all the gems in the bundle

SYNOPSIS
       bundle list [--name-only] [--paths] [--without-group=GROUP] [--only-group=GROUP]

(...)

$ bundle list --only-group=development
Unknown switches '--only-group=development'
```

So, instead, I enable the new list command _always_ and remove the
`bundle list => bundle show` alias.

They are actually compatible.