fs_mgr: overlayfs: erofs is in staging

erofs is in upstream staging and in 4.19, and we see some Android
partners considering it as an option instead of squashfs.

However, squashfs and erofs filesystems are always 100% full, so
we can safely remove the string checking instead and just rely on
the statvfs call.  We will provide lip-service to our support in
comment only.

This does _not_ mean we support the decision by our partners to use
erofs, as we have not evaluated the performance.

Test: adb_remount_test.sh
Bug: 109821005
Change-Id: I80ea3a33b9da6684ee95489132bc8a88b2310ac8