Revert "man/Makefile.am: Add --sgml to docbook2man invocation"
Revert "man/Makefile.am: Add --sgml to docbook2man invocation"

This reverts commit b4143665fe46ad71379de41441dd36cd6be39066.

docbook-utils' docbook2man doesn't have a --sgml flag
and is arguably a more modern utility compared to docbook2man
from docbook2X
superm1/fwupd,hughsie/fwupd,fwupd/fwupd,superm1/fwupd,vathpela/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd,superm1/fwupd,vathpela/fwupd,vathpela/fwupd,vathpela/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd