Update the comment for obsolete symlink handling .

Symlink is a filebased OTA feature, and the corresponding updater
function has been removed in
https://android-review.googlesource.com/#/c/350357/.

Also the only place where we call "package_extract_dir()" is to unpack
some bootloader dir in vendor's code. We plan to remove it also in a
separate bug.

Bug: 31917448
Test: mma
Change-Id: I3986d60958e64e0d5d8fa5f5bd508c579fb7fa2c