functional-geometry : Remove obsolete keyword used in define-command
functional-geometry : Remove obsolete keyword used in define-command

AFAIK, `:provide-default nil` is used in Genera's Dynamic Windows if
the default value is boolean (can be nil). It's no longer mentioned in
CLIM 2 Specification and functionally equivalent of not providing
`:defualt` keyword. Fixes #543.

slyrus/mcclim,slyrus/mcclim