zpool-features.5: remove "booting not possible with this feature"s

The exact limitations on what features are supported when booting
vary considerably depending on the environment. In order to minimize
confusion avoid categorical statements which assume GRUB2 is being
used. The supported GRUB2 features are covered earlier in this man
page for easy reference.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11842