fix: remove unused root variable from commit-msg hook

The `commit-msg` hook never referenced `${root}`. Touching
the file for the worktree fix surfaced the pre-existing SC2034
warning; remove the dead line rather than carrying it forward.

Author: batpigandme

tools/git/hooks/commit-msg

@@ -33,9 +33,6 @@ skip_lint="${SKIP_LINT_COMMIT}"
 # Get the path to a file containing the commit message:
 commit_message="$1"
 
-# Determine root directory:
-root=$(git rev-parse --show-toplevel)
-
 # Determine git directory (resolves where `${root}/.git` is file OR directory):
 git_dir=$(git rev-parse --absolute-git-dir)