fs_mgr: remove by_name_symlinks_map_ from AvpOps

With the addition of the /dev/block/by-name/ symlinks created for the
boot_device, we no longer need to use a map to track the symlinks for
the partitions AVB needs to access.

This will help us in removing the requirement to specify which
partitions contain AVB metadata.

Bug: 117933812
Test: boot blueline_mainline with AVB
Change-Id: I1d46dba5b2fc16b2a14f861b34225ac0f2995b60