Fixed warning about aborted connection in tests

refs https://github.com/TryGhost/Toolbox/issues/389

- if we enable warning logs in E2E tests, we get a bunch of error
  messages saying `ERROR Unhandled rejection: aborted` coming from the
  SQLite DB reset code
- specifically, it's coming from the line that resets the DB by copying
  the file
- this line was initially added because we would see random SQLite
  "malformed database" errors
- I have a feeling that was due to something else, but I can't be sure
- I'm also not sure how else we should shut the DB connection, as this
  is the recommended way but it throws an unhandled rejection
- this commit is a bit of a gamble because I'm not actually sure what
  was causing the problem, but it gets rid of the errors locally and
  doesn't regress on the random failures

ghost/core/test/utils/db-utils.js

@@ -48,7 +48,6 @@ module.exports.reset = async ({truncate} = {truncate: false}) => {
         const filenameOrig = `${filename}-orig`;
 
         if (dbInitialized) {
-            await db.knex.destroy();
             await fs.copyFile(filenameOrig, filename);
         } else {
             await fs.remove(filename);