Project file: Remove reference to an extension file that no longer exists

[mindgraffiti]

Fixes # n/a

This PR removes a file reference that no longer exists in the project.

To test

Doesn't really require testing. Build and run the project.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[mindgraffiti]

Thank you @ScoutHarris!