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