rxgen: Remove pointless assignment

The value we assign to defp is never used (it's almost immediately
overwritten), and it is guaranteed to be the same as the existing
value. So, just remove the assignment.

Caught by clang-analyzer

Change-Id: I362fe08c623b5046f2c8b0b16c42dcfa12009981
Reviewed-on: http://gerrit.openafs.org/9162
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>