Remove unused code

It seems that ASP::Processor#process calls #on_if (and not #on_unless)
when encountering an `unless` node.
Remove unused method

This method was left behind after commit 623a886862f15083672057e4fe7a.
Remove unused method
Spec for invalid configuration of Style/Next
Spec for invalid configuration of GuardClause
Spec for invalid config of SpaceInsideBlockBraces
Merge pull request #1535 from bquorning/remove-unused-code

Remove unused code
Merge pull request #1534 from bquorning/spec-invalid-configuration

Spec invalid configuration
Test EmptyLinesAroundClassBody on singleton class
Spec for Token#to_s

Token#to_s was introduced in https://github.com/bbatsov/rubocop/pull/8
and seems untested forever. I assume it was introduced for debugging
purposes.
Test RedundantSelf with class methods, too
Test LiteralInCondition on post-loop while/until
Spec for passing an ambiguous formatter name
Don't check if the configuration option is nil

The default value for AllowedVariables (per default.yml) is `[]`.