INSTALL: remove discussion of SHA-1 backends

INSTALL: remove discussion of SHA-1 backends

The claim that OpenSSL is the default SHA-1 backend hasn't been true
since e6b07da2780 (Makefile: make DC_SHA1 the default, 2017-03-17),
but more importantly tweaking the SHA-1 backend isn't something that's
common enough to warrant discussing in the INSTALL document, so let's
remove this paragraph.

This discussion was originally added in c538d2d34ab (Add some
installation notes in INSTALL, 2005-06-17) when tweaking the default
backend was more common. The current wording was added in
5beb577db8c (INSTALL: Describe dependency knobs from Makefile,
2009-09-10).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>