Remove validate support when reusing cached pack

Remove validate support when reusing cached pack

Cached packs are only used when writing over the network or to
a bundle file and reuse validation is always disabled in these
two contexts. The client/consumer of the stream will be SHA-1
checksumming every object.

Reuse validation is most critical during local GC to avoid silently
ignoring corruption by stopping as soon as a problem is found and
leaving everything alone for the end-user to debug and salvage.
Cached packs are not supported during local GC as the bitmap rebuild
logic does not support including a cached pack in the result.

Strip out the validation and force PackWriter to always disable the
cached pack feature if reuseValidation is enabled.

Change-Id: If0d7baf2ae1bf1f7e71bf773151302c9f7887039