Remove unused class SGEnviro

Remove unused class SGEnviro

This class and their source files visual_enviro.[h|c]xx were
unused since OSG transition. It's only functionality was to
keep a variable for the enable-state of rendering of precipitation.
This has now been moved to SGPrecipitation.
This patch also removes all references to this class and
visual_enviro.hxx. Wxradar needs a new implementation for cloud
echoes which probably should be part of fgclouds.

See also SimGear commit 4e6159aa07f2817c09fad72db64d713e47f6f3da