rubocop: Move AllowedMethods inheritance to the cop it's used in

- `Include` and `Exclude` are ubiquitous but `AllowedMethods` less so.
- Move the inheritance to the single cop configuration that it's used
  in. Like we do with some other cop configs where they have default
  paths/settings that we want to add to, not replace.

Library/.rubocop.yml

@@ -10,7 +10,6 @@ inherit_mode:
   merge:
     - Include
     - Exclude
-    - AllowedMethods
 
 AllCops:
   TargetRubyVersion: 2.6

Library/Homebrew/.rubocop.yml

@@ -16,6 +16,9 @@ Layout/MultilineMethodCallIndentation:
     - "**/*_spec.rb"
 
 Naming/PredicateName:
+  inherit_mode:
+    merge:
+      - AllowedMethods
   AllowedMethods:
     - is_32_bit?
     - is_64_bit?