R1:30c47690ffe3

Remove unused *ParallelQueries() database connection methods

Summary:
`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

Test Plan: Grep for both functions.