fs_mgr: Refactor by inlining trivial helpers

Inline some trivial helpers and remove unused header declarations.

Remove fs_mgr_*access() as it is really just access().
Remove fs_mgr_overlayfs_super_device() as we always want the primary
slot and having this wrapper isn't particularly helpful.

Test: adb-remount-test
Change-Id: I2581fd7c7d5071cbb97778535b7811dbcb80d76e