wifi: mac80211: remove support for AddBA with fragmentation

HE added support for dynamic fragmentation inside aggregation
sessions, but no existing driver ever advertises it. Thus,
remove the code for now, it cannot work as-is in MLO. For it
to properly work in MLO, we'd need to validate that the frag
level is identical across all the link bands/iftypes, which
is a good amount of complex code that's just not worth it as
long as no driver has support for it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>