Remove snyk workflow

This repo has only configuration, without any code. Therefore, the scans
do not work as intended, causing the workflow to fail.

.github/workflows/ci.yml

@@ -11,17 +11,6 @@ on:
         type: string
 
 jobs:
-  snyk-security:
-    name: SNYK security analysis
-    uses: alphagov/govuk-infrastructure/.github/workflows/snyk-security.yml@main
-    with:
-      skip_sca: true
-    secrets: inherit
-    permissions:
-      contents: read
-      security-events: write
-      actions: read
-
   codeql-sast:
     name: CodeQL SAST scan
     uses: alphagov/govuk-infrastructure/.github/workflows/codeql-analysis.yml@main