Mistake with mpn_sqr_n

[Torbjorn Granlund]

After advice from Niels and Marco, I went ahead and removed the spurious
documentation.  The result is in the GMP mercurial repo.

I think we should make a final 4.3, i.e. 4.3.4, soon.  The manual will
be corrected, and there will be a few minor bug fixes.  And then a few
days later we might release 5.0.2, also fixing some minor bugs.

(It turned out to be even more odd than I thought.  The mpn_sqr_n
function existed until 4.3.1, where it was undocumented, but is called
mpn_sqr in GMP 4.3.2.  But apparently, somebody started using it, and
this person must have used a new manual but linked to an old library.)

-- 
Torbjörn