Revert "vmimage.subr: pkg autoremove after pkg install"

This reverts commit 6a13aeac3c1f98db7cf156f24a4d6bc8d3c321f6.

The "bogus dependencies being installed" issue was traced down to pkg
confusion surrounding libclang_rt.asan-i386.so linking to a 32-bit
libgcc_s.so.1, and a more minimal workaround of "install clibs-lib32"
has been applied, so this hack is no longer required.

MFC after: 3 days
Sponsored by: Amazon

(cherry picked from commit 4b29bf721ef09c399f3e4a4ab5ca9f549c5b1550)