Remove MK_AUTO_OBJ from env passed to PORTS_MODULES

This fixes a failure to resolve object file paths seen when buildkernel
(which sets MK_AUTO_OBJ=yes) and installkernel (which sets MK_AUTO_OBJ=no)
are run as separate steps. r329232 partially fixed this scenario by removing
MAKEOBJDIR, but it seems the AUTO_OBJ setting also needs to be on the same
page for the build and install steps.

Reviewed by: bdrewery
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14143