conky: Don't enable nvidiaSupport by default

This has been introduced in 6a6fb6d3

.

Relying on non-free software by default is probably a bad idea. Apart
from the fact that (sane) people usually don't want to have it sitting
on their system even people who don't care will have to set
"allowUnfree" to true in order to install conky.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @canndrew, @Mic92