libafs: cleanup redundant assignment

Cleanup redundant clearing of the avcp pointer in lookup, which
looks to be a cut and paste error from the ibm afs days.

Change-Id: Ib554d862bd8485be2a4dbeb38fcaacac57c1e818
Reviewed-on: http://gerrit.openafs.org/9085
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>