g_part_ebr: always create "compat" aliases

The "canonical" EBR partition names like ada0s4+00002081 are not
particularly meaningful. The "compat" aliases share the same namespace
as the parent MBR, resulting in user-friendly names like ada0s6.
These names are consistent with the way Linux names EBR partitions.

We previously provided a sysctl kern.features.geom_part_ebr_compat
(enabled by default) to control the "compat" names. Remove the sysctl
and always create the aliases.

Relnotes: yes
Reviewed by: cem, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38812

(cherry picked from commit 6c7bc93da699054bb8ead58fdc226e084d929515)