manpage-bold-literal.xsl: stop using git.docbook.backslash

manpage-bold-literal.xsl: stop using git.docbook.backslash

We used to assign git.docbook.backslash one of two different values --
one "normal" and one for working around a problem with docbook-xsl 1.72.
After the previous commit, we don't support that version anymore and
always use the "normal" value, a literal backslash.

Just explicitly use a backslash instead of using git.docbook.backslash.
The next commit will drop the definition of git.docbook.backslash
entirely.

Signed-off-by: Martin Ågren <redacted>
Signed-off-by: Junio C Hamano <redacted>