There are several places in the code which implements
logic of decoding byte array into Envelope, there is
method available inside txutils.go called GetEnvelopeFromBlock,
there is GetEnvelope from proputils.go and there is
ExtractEnvelope from commonutils.go.

This commits removes duplicate and switching to reuse single
method from txutils.go.

Change-Id: Ia7a963cfd0df85377ef4294d2dba33cca1eb904b
Signed-off-by: Artem Barger <bartem@il.ibm.com>