releasetools: Update the comments regarding selinux_fc.

The former comment no longer applies, as we have been always packing
META/file_contexts.bin in a target_files.zip (commit aa7318c3, since
Nougat), and we no longer look for the one under BOOT/RAMDISK/ (commit
d14b8956, since Q).

Test: N/A
Change-Id: I03f361234bf440e942f21e5a624862590248544b