(stable) When proxying HTTP repository responses from repository nodes, discard content description headers

Summary:
Ref T13517. See that task for details about the underlying issue here.

Currently, we may decode a compressed response, then retransmit it with leftover "Content-Encoding" and "Content-Length" headers. Instead, strip these headers.

Test Plan:

• In a clustered repository setup, cloned a Git repository over HTTP.
• Before: Error while processing content unencoding: invalid stored block lengths
• After: Clean clone.

Maniphest Tasks: T13517

Differential Revision: https://secure.phabricator.com/D21167