Delete checkObjectCollisions from PackParser

Delete checkObjectCollisions from PackParser

This flag was added to provide an unsafe operation on the local
repository because the storage.dht code was too damn slow to provide
proper safe Git behavior all of the time. Now that stoarge.dht has
been removed from the repository, also delete this unsafe flag to
prevent applications from misusing the JGit library and permitting
users to potentially damage their local repository with bad data
received from an untrusted peer.

Change-Id: Ib1861c48bb74836731e7b7d57b635dd654b0dc66
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>