ktls.4: Remove an obsolete statement

[markj]

Summary

The default mb_use_ext_pgs value was toggled in commit 52cd25eb1aa.

[jhb]

Hmm, not sure if we want to keep documenting the requirement and just omit the comment about them being off by default? Maybe it's not worth doing though.

[markj]

In D30393#682763, @jhb wrote:

Hmm, not sure if we want to keep documenting the requirement and just omit the comment about them being off by default? Maybe it's not worth doing though.

I'm not sure either. I feel like mb_use_ext_pgs is effectively a chicken switch, so I'm not sure there's much point in documenting it: if you changed it, you (hopefully) already know what you're doing.