Auto merge of #6446 - bundler:segiddins/remove-unused-ivar, r=colby-s…

…wandale

[SpecGroup] Remove unused ivar

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

The problem was this ivar was unused.

### What was your diagnosis of the problem?

My diagnosis was we should remove it. For performance, you know.

lib/bundler/resolver/spec_group.rb

@@ -14,7 +14,6 @@ def initialize(all_specs)
         @version = exemplary_spec.version
         @source = exemplary_spec.source
 
-        @required_by = []
         @activated_platforms = []
         @dependencies = nil
         @specs        = Hash.new do |specs, platform|