ci: drop skip-if-draft job (#13961)

It's been effectively disabled for half a year already, and according to
internal discussion we don't want it back.

substrate/scripts/ci/gitlab/skip_if_draft.sh

@@ -1,14 +0,0 @@
-#!/bin/sh
-url="https://api.github.com/repos/paritytech/substrate/pulls/${CI_COMMIT_REF_NAME}"
-echo "[+] API URL: $url"
-
-draft_state=$(curl -H "Authorization: token ${GITHUB_PR_TOKEN}" "$url" | jq -r .draft)
-echo "[+] Draft state: $draft_state"
-
-if [ "$draft_state" = 'true' ]; then
-  echo "[!] PR is currently a draft, stopping pipeline"
-  exit 1
-else
-  echo "[+] PR is not a draft. Proceeding with CI pipeline"
-  exit 0
-fi