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.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15919

Differential Revision: https://we.phorge.it/D25789