chore: remove stray .worktrees/fix-pr-2307-ci gitlink

This was an accidentally-committed gitlink (160000 mode) with no
corresponding entry in .gitmodules. It does not break git itself
but causes any cargo git-dep consumer to fail cloning the repo:

  failed to update submodule `.worktrees/fix-pr-2307-ci`
  no URL configured for submodule '.worktrees/fix-pr-2307-ci'

Same class of residue that 837b5a0 on feat/exclude-constraint-upstream
cleaned up, but that commit never reached main so the gitlink still
ships on every descendant branch.

Author: fmguerreiro