Retire 'stupid' merge strategy

As pointed out by Linus, this strategy tries to take the best merge
base, but 'recursive' just does it better. If one needs something more
than 'resolve' then he/she should really use 'recursive' and not
'stupid'.

Cf. Message-ID: <alpine.LFD.1.10.0807030947360.18105@woody.linux-foundation.org>

Signed-off-by: Junio C Hamano <gitster@pobox.com>