vmimage.subr: autoremove -y

Running 'pkg autoremove' without -y results in VM image builds failing
when (bogusly installed) packages are removed.

Approved by: re (cperciva)
Fixes: 6a13aeac3c1f ("vmimage.subr: pkg autoremove after pkg install")
MFC after: immediately (needed for 15.0-RC1)

(cherry picked from commit 509dfd369046ee45b724d119c9eed43228fbb894)
(cherry picked from commit 9600598a5c011c161813f929be53c3748a6a0f01)