chore(package): remove preinstall only-allow pnpm check

Remove the preinstall script that enforces pnpm usage. This check
is unnecessary as the project uses nix develop which provides
the correct pnpm version, and it can cause issues in CI environments.

Author: ryoppippi

package.json

@@ -45,7 +45,6 @@
 		"lint:oxfmt": "oxfmt --no-error-on-unmatched-pattern --check .",
 		"lint:oxlint": "oxlint --max-warnings=0 --type-aware --type-check",
 		"lint:knip": "knip",
-		"preinstall": "npx only-allow pnpm",
 		"prepack": "npm pkg delete scripts.preinstall && pnpm run build",
 		"test": "vitest",
 		"coverage": "vitest run --coverage"