Remove use_marker_ranges feature_flag

What does this MR do and why?

Removes use_marker_ranges feature_flag.

This flag has been enabled for a while: #324638 (comment 550455214)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #391508 (closed)

lib/gitlab/diff/highlight.rb

@@ -130,12 +130,6 @@ def blob_highlighting(diff_line)
         end
       end
 
-      # Deprecated: https://gitlab.com/gitlab-org/gitlab/-/issues/324638
-      # ------------------------------------------------------------------------
-      def inline_diffs
-        @inline_diffs ||= InlineDiff.for_lines(@raw_lines)
-      end
-
       def old_lines
         @old_lines ||= highlighted_blob_lines(diff_file.old_blob)
       end