`executeParallelQueries()` is never called as `supportsParallelQueries()` returns `false` both in `AphrontDatabaseConnection` and `AphrontMySQLDatabaseConnection`.
Thus remove this unused code which was intentionally meant for HHVM which does not support PHP>=7 anyway.
P.S. Dear Facebook, if you are using this, let us know. We don't want to hurt your legacy Phabricator installation.
Closes T15919