Remove dead code from plugin scripts

canonical_widget_id was a pure pass-through, making its reserved-id
check a tautology; require_omarchy_path and choose_value were defined
but never called.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bin/omarchy-plugin

@@ -54,10 +54,6 @@ fail() {
   exit 1
 }
 
-require_omarchy_path() {
-  [[ -n ${OMARCHY_PATH:-} ]] || fail "OMARCHY_PATH is not set"
-}
-
 require_command() {
   command -v "$1" >/dev/null 2>&1 || fail "$1 is required"
 }