Delete legacy config

Maintaining a separate "edge" config and "legacy" config probably served
us well [back around the time we were migrating to rubocop
0.68](github@d533765),
but it's not working well for us anymore. Things changed often before
rubocop 1.0, and there are versions out there that don't work perfectly
with either of these configs (see
github#86, for example).

This commit sets a version constraint for rubocop ">= 1.0.0" and deletes
the `*_deprecated.yml` files that only worked with rubocop <
0.68. With the deprecated files deleted, it also consolidates the
remaining files into default.yml and rails.yml.

Gemfile.lock

@@ -2,7 +2,7 @@ PATH
   remote: .
   specs:
     rubocop-github (0.17.0)
-      rubocop
+      rubocop (>= 1.0.0)
       rubocop-performance
       rubocop-rails
 
@@ -48,7 +48,7 @@ GEM
     rake (13.0.6)
     regexp_parser (2.2.0)
     rexml (3.2.5)
-    rubocop (1.25.0)
+    rubocop (1.25.1)
       parallel (~> 1.10)
       parser (>= 3.1.0.0)
       rainbow (>= 2.2.2, < 4.0)

README.md

@@ -4,28 +4,6 @@ This repository provides recommended RuboCop configuration and additional Cops f
 
 ## Usage
 
-Rubocop 0.68 removed performance cops and 0.72 removed Rails cops. However, upgrading `rubocop-github` without modification will almost definitely create very many new offenses. The current version of this gem exposes the "legacy" configuration under `config/default.yml` and `config/rails.yml` which should be used *if and only if* the version of rubocop is locked to `< 0.68` in your project (which it should be unless you `bundle update rubocop`). It also exposes an "edge" configuration under `config/default_edge.yml` and `config/rails_edge.yml` so that the changes can be tested without introducing breaking changes.
-
-### Legacy usage
-
-**Gemfile**
-
-``` ruby
-gem "rubocop", "< 0.68"
-gem "rubocop-github"
-```
-
-**.rubocop.yml**
-
-``` yaml
-inherit_gem:
-  rubocop-github:
-    - config/default.yml
-    - config/rails.yml
-```
-
-### Edge usage
-
 **Gemfile**
 
 ``` ruby
@@ -43,6 +21,11 @@ inherit_gem:
     - config/rails_edge.yml
 ```
 
+### Legacy usage
+
+If you are using a rubocop version < 1.0.0, you can use rubocop-github version
+0.16.2 (see the README from that version for more details).
+
 ## Testing
 
 `bundle install`