The filer-side mount peer registry (tier 1 of peer chunk sharing) was
gated behind -mount.p2p (default true). Idle cost is negligible — a
tiny in-memory map plus a 60s sweeper — so the opt-out is not worth the
surface area.
Removes the flag from weed filer, weed server (-filer.mount.p2p), and
weed mini, and always constructs the registry in NewFilerServer. Also
drops the now-dead nil guards in MountRegister/MountList/sweeper and
the TestMountRegister_DisabledIsNoOp case.