Remove obsolete comment from cmds/installd/otapreopt_chroot.cpp.

APEX packages are deactivated at the end of `otapreopt_chroot`, so
loop devices used to mount them are returned to the system.

Test: m otapreopt_chroot
Bug: 113373927
Change-Id: I8b55787ffa7abdfe63f9778623f5e2317b8a4525