Remove outdated "rebuild_summaries.php" script

Summary:
Ref T13195. See PHI842. Alternative to D19638.

Instead of doing all the stuff in D19638, //just// remove the `rebuild_summaries.php` script. This script is outdated, copy/pastes the rebuild logic, and doesn't understand unreachable commits.

If we had some use for it it should move to `bin/repository rebuild-summary ...` or similar, but it's not clear there's any use for it. The incremental summary rebuilds seem to work fine as-is.

Test Plan: Grepped for callers or documentation referencing this script, found nothing.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13195

Differential Revision: https://secure.phabricator.com/D19643

Author: epriestley