Getting rid of LSEG.m

I noticed a Coverity complaint about the "m" field of LSEG being
uninitialized, which indeed it is because all the places that
would set it are ifdef'd out. So why didn't the field itself
get ifdef'd out as well?

Or perhaps we should just remove both the field and the ifdef'd
assignments. That's a bit more drastic but I can't really see
this code ever coming back to life ... especially since the notion
of a field that's not stored on disk but is valid in in-memory
copies seems impossibly error-prone. Most functions can have no
idea whether their input is residing in a disk buffer or not.
And adding the bookkeeping to determine that would surely cost
more than just recomputing the slope when needed.

regards, tom lane