xen/grant-table: Remove outdated warning in gnttab_grow_table()

xen/grant-table: Remove outdated warning in gnttab_grow_table()

One of the warning message in gnttab_grow_table() refers to a function
was removed in commit 6425f91c72 "xen/gnttab: Fold grant_table_{create,
set_limits}() into grant_table_init()".

Since the commit, gt->active will be allocated while initializing the
grant table at domain creation. Therefore gt-active will always be
valid.

Rather than replacing the warning by another one, drop the check
completely as we will likely not come back to a semi-initialized world.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>