update_engine: Remove zlib fingerprint checking

This is not needed as imgdiff is not being used and will be replaced by
puffin.
Presence of this code makes updating to newer version of zlib
impossible. (e.g. 1.2.8->1.2.11 update introduces some changes in
compression algorithm which make compressed output different).

BUG=None
TEST="ebuild zlib.ebuild test"

Change-Id: I50793f552a44faaba5771271fdcf2b4f4aeba6e4
Reviewed-on: https://chromium-review.googlesource.com/522949


Commit-Ready: Andrey Ulanov <andreyu@google.com>
Tested-by: Andrey Ulanov <andreyu@google.com>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>