docs/conf.py: clean up unset variables left from template generation
docs/conf.py: clean up unset variables left from template generation

We do not intend to build anything but HTML (and probably PDF), thus most settings are
not relevant or even redundant. Also remove other settings that do not
differ from their default value.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,ejoerns/rauc,jluebbe/rauc,jluebbe/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc