g_part_ebr: always create "compat" aliases

[emaste]

Summary

The "canonical" ebr names like `ada0s4+00002081` are not particularly
meaningful.  The "compat" aliases share the same namespace as the parent
MBR, resulting in names like `ada0s6`.  This is consistent with the way
Linux names partitions within an EBR.

We previously provided a sysctl kern.features.geom_part_ebr_compat,
which defaulted to enabled.  Remove the sysctl and always create the
"compat" aliases.

Relnotes: yes
Sponsored by: The FreeBSD Foundation

[emaste]

Prompted in part by investigation after reading https://www.theregister.com/2022/05/20/freebsd_131/

[cem]

Lgtm.