Remove full_namespace_format_regex because not used

Beside that it's using the same instance variable with
`namespace_format_regex` which is going to cause issues.

lib/gitlab/path_regex.rb

@@ -185,10 +185,6 @@ module Gitlab
       @full_project_git_path_regex ||= %r{\A\/?(?<namespace_path>#{full_namespace_route_regex})\/(?<project_path>#{project_route_regex})\.git\z}
     end
 
-    def full_namespace_format_regex
-      @namespace_format_regex ||= /A#{FULL_NAMESPACE_FORMAT_REGEX}\z/.freeze
-    end
-
     def namespace_format_regex
       @namespace_format_regex ||= /\A#{NAMESPACE_FORMAT_REGEX}\z/.freeze
     end