Deleted custom-build workflow

- this is no longer needed because we added support to build from a
  branch

.github/workflows/custom-build.yml

@@ -1,21 +0,0 @@
-name: Custom build
-on:
-  workflow_dispatch:
-    inputs:
-      branch:
-        description: 'Branch to build from'
-        type: string
-        required: false
-        default: 'main'
-      version:
-        description: 'Version to build'
-        type: string
-        required: true
-env:
-  FORCE_COLOR: 1
-jobs:
-  custom:
-    uses: tryghost/actions/.github/workflows/custom.yml@main
-    with:
-      branch: ${{ inputs.branch }}
-      version: ${{ inputs.version }}