ci: drop the deprecated GITLEAKS_LICENSE secret mapping (#637)

Syncs the template change from netresearch/.github#330.

Secret scanning runs on **betterleaks**, which is OSS and needs no
license.
The shared reusable declares `GITLEAKS_LICENSE` only for backwards
compatibility and **never reads it**, so forwarding a repo secret into
it was
dead plumbing that widened the secret exposure surface for no benefit.

The file was byte-identical to the previous template revision before
this
change, so the diff is exactly the removed `secrets:` mapping — no
repo-specific drift was touched. Required to keep `check-template-drift`
green.

Author: CybotTM

.github/workflows/gitleaks.yml

@@ -14,5 +14,3 @@ jobs:
     permissions:
       contents: read
       security-events: write
-    secrets:
-      GITLEAKS_LICENSE: ${{ secrets.GITLEAKS_LICENSE }}