libflake: Remove unused maybeParseFlakeRef and maybeParseFlakeRefWith… (backport #13406)

[mergify]

…Fragment

Motivation

These wrappers don't seem to be used anywhere in and out of tree. Also the declaration in the header has an incorrect function name maybeParseFlake.

Context

Closes #11948

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This is an automatic backport of pull request #13406 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 75412eb has failed:

On branch mergify/bp/2.29-maintenance/pr-13406
Your branch is up to date with 'origin/2.29-maintenance'.

You are currently cherry-picking commit 75412ebc3.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libflake/flakeref.cc
	both modified:   src/libflake/include/nix/flake/flakeref.hh

no changes added to commit (use "git add" and/or "git commit -a")

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