singlehtml: deprecate 'fix_refuris' helper function

[jayaddison]

Feature or Bugfix

• Deprecation

Purpose

• Deprecates a singlehtml builder helper method that I believe should be redundant since #11970 and #12551 are in place, but that might, perhaps, be re-used by other functionality (extensions / codebases).

Detail

• Deprecates the sphinx.builders.singlehtml.SingleFileHTMLBuilder.fix_refuris method, and adds a warning noting that it's planned for removal in Sphinx 9 (the next major version).

Relates

• Follow-up to #11970 and #12551.

Edit: update description: this PR removes calls to the method, so don't imply that this is deprecation of already-unused code.