Doc: drop support for docbook-xsl before 1.71.1

Doc: drop support for docbook-xsl before 1.71.1

Drop the DOCBOOK_SUPPRESS_SP mechanism, which needs to be used with
docbook-xsl versions 1.69.1 through 1.71.0.

We probably broke this for Asciidoctor builds in f6461b82b9
("Documentation: fix build with Asciidoctor 2", 2019-09-15). That is, we
should/could fix this similar to 55aca515eb ("manpage-bold-literal.xsl:
match for namespaced "d:literal" in template", 2019-10-31). But rather
than digging out such an old version of docbook-xsl to test that, let's
just use this as an excuse for dropping this decade-old workaround.

DOCBOOK_SUPPRESS_SP was not needed with docbook-xsl 1.69.0 and older.
Maybe such old versions still work fine on our docs, or maybe not. Let's
just refer to everything before 1.71.1 as "not supported". The next
commit will increase the required/assumed version further.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>