Retire ZFS_TEARDOWN_TRY_ENTER_READ

There were never any users and it so happens the operation is not even
supported by rrm locks -- the macros were wrong for Linux and FreeBSD
when not using it's RMS locks.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Closes #13906