vmm: Remove an unneeded initialization of "retu"

vm_handle_ipi() unconditionally initializes "retu". No functional
change intended.

Reviewed by: jhb, corvink
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38446

(cherry picked from commit ba34de1b3bf965d15f6c76c97fa7e6d17194d401)