Revert yarn.lock addition to .gitignore

Removing yarn.lock from .gitignore as it should be tracked when present.

Author: dduugg

.gitignore

@@ -72,9 +72,6 @@ web_modules/
 # Yarn Integrity file
 .yarn-integrity
 
-# Yarn lock file (project uses npm, not yarn)
-yarn.lock
-
 # dotenv environment variable files
 .env
 .env.development.local