Eliminate useless double-bolding in man pages

As far as I know and can tell from testing, \fB\fB...\fR\fR is exactly
equivalent to \fB...\fR.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8641