tweak jest e2e config

`maxConcurrency` is how many `test.concurrent` to run at once, nothing
to do with how many test *files* to run (that's probably `maxWorkers`
which is already 1)

packages/backend/jest.config.e2e.cjs

@@ -12,5 +12,4 @@ module.exports = {
 	testMatch: [
 		"<rootDir>/test/e2e/**/*.ts",
 	],
-	maxConcurrency: 1, // these tests share a database, running more than one at a time is a bad idea
 };