Remove overlooked sun_attr based macros

The NORETURN, CONST, and PURE macros in the FreeBSD platform
code were based on the sun_attr__ macro which was removed in
commit 5dbf6c5a6. This caused a build failure because the
NORETURN macro was still used in one place in kernel code.
The CONST and __PURE macros were entirely unused.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12435