Drop the rubygems-specific lockfile tokenizer and parser sources

These files implemented the read path that Bundler::LockfileParser now
covers via the adapter in Gem::RequestSet#load_lockfile. The Lockfile
class itself is kept because Gem::RequestSet#install_from_gemdeps still
uses Lockfile.build / Lockfile#write for emitting .lock files, which
will be migrated separately.

Author: hsbt

lib/rubygems/request_set.rb

@@ -507,4 +507,3 @@ def tsort_each_child(node) # :nodoc:
 
 require_relative "request_set/gem_dependency_api"
 require_relative "request_set/lockfile"
-require_relative "request_set/lockfile/tokenizer"

lib/rubygems/request_set/lockfile.rb

@@ -231,5 +231,3 @@ def requests
     @set.sorted_requests
   end
 end
-
-require_relative "lockfile/tokenizer"