Gemfile: Remove Gemfile.local support

We don't use a Gemfile.local in this repository and dependabot fails to
parse it.

Author: bastelfreak

Gemfile

@@ -23,5 +23,3 @@ group(:development, optional: true) do
 end
 
 #gem 'rubocop', "~> 0.34.2"
-
-eval_gemfile("#{__FILE__}.local") if File.exist?("#{__FILE__}.local")