Remove optional project_id field (#387)

Now that
#383 is
merged, we can make this optional.

Author: sethvargo

.github/workflows/integration.yml

@@ -53,7 +53,6 @@ jobs:
         id: 'deploy'
         uses: './'
         with:
-          project_id: '${{ vars.PROJECT_ID }}'
           working_directory: '${{ github.workspace }}/example-app'
           build_env_vars: |-
             FOO=bar