Remove dead constants and unused AnonClass (#1642)

## Summary

- Remove `RDoc::AnonClass` class and file (documented as "never used")
- Remove `Darkfish::VERSION` (unreferenced)
- Remove `Formatter::InlineTag` struct (unused after inline parser
rewrite)
- Remove `ToHtmlCrossref::CLASS_REGEXP_STR` and `METHOD_REGEXP_STR`
(aliased but unused within the file)

### Process & tools used

1. **Detect candidates** — Used `detect_dead_constants` from
[Rubydex](https://github.com/Shopify/rubydex) to find constants with
zero resolved references in `lib/`. Got 22 candidates.
2. **Filter out false positives** — Grepped for actual usage of each
candidate, catching private constants used internally, dynamic access
via `const_defined?`/`const_get`, default parameters, and generated code
(Racc).
3. **Remove confirmed dead code** — Deleted 5 constants and 1
class+file.
4. **Run full test suite** — Caught that one deletion (`RI::Formatter`)
broke a `require_relative` in `ri/driver.rb`, so restored it. Final run:
2384 tests, 0 failures.

Author: st0012

lib/rdoc.rb

@@ -189,7 +189,6 @@ def self.home
   autoload :Context,        "#{__dir__}/rdoc/code_object/context"
   autoload :TopLevel,       "#{__dir__}/rdoc/code_object/top_level"
 
-  autoload :AnonClass,      "#{__dir__}/rdoc/code_object/anon_class"
   autoload :ClassModule,    "#{__dir__}/rdoc/code_object/class_module"
   autoload :NormalClass,    "#{__dir__}/rdoc/code_object/normal_class"
   autoload :NormalModule,   "#{__dir__}/rdoc/code_object/normal_module"

lib/rdoc/code_object.rb

@@ -10,7 +10,6 @@
 # * RDoc::Context
 #   * RDoc::TopLevel
 #   * RDoc::ClassModule
-#     * RDoc::AnonClass (never used so far)
 #     * RDoc::NormalClass
 #     * RDoc::NormalModule
 #     * RDoc::SingleClass

lib/rdoc/code_object/anon_class.rb

@@ -73,11 +73,6 @@ class RDoc::Generator::Darkfish
       css/rdoc.css
   ]
 
-  ##
-  # Release Version
-
-  VERSION = '3'
-
   ##
   # Description of this generator
 

lib/rdoc/generator/darkfish.rb

@@ -14,13 +14,6 @@
 
 class RDoc::Markup::Formatter
 
-  ##
-  # Tag for inline markup containing a +bit+ for the bitmask and the +on+ and
-  # +off+ triggers.
-
-  InlineTag = Struct.new(:bit, :on, :off)
-
-
   ##
   # Converts a target url to one that is relative to a given path
 

lib/rdoc/markup/formatter.rb

@@ -8,9 +8,7 @@ class RDoc::Markup::ToHtmlCrossref < RDoc::Markup::ToHtml
 
   # :stopdoc:
   ALL_CROSSREF_REGEXP = RDoc::CrossReference::ALL_CROSSREF_REGEXP
-  CLASS_REGEXP_STR    = RDoc::CrossReference::CLASS_REGEXP_STR
   CROSSREF_REGEXP     = RDoc::CrossReference::CROSSREF_REGEXP
-  METHOD_REGEXP_STR   = RDoc::CrossReference::METHOD_REGEXP_STR
   # :startdoc:
 
   ##