Remove box-model rule.

Most projects should not have to support IE6. The unfortunate ones should reenable the rule in the local build.properties file.

build.default.properties

@@ -49,7 +49,7 @@ csslint.repository.revision = v0.8.5
 
 # The csslint rules to use
 # The current selection is based on discussion here: http://mattwilcox.net/archive/entry/id/1054/
-csslint.rules = box-model,display-property-grouping,duplicate-properties,empty-rules,known-properties,box-sizing,compatible-vendor-prefixes,gradients,vendor-prefix,import,zero-units,shorthand,important
+csslint.rules = display-property-grouping,duplicate-properties,empty-rules,known-properties,box-sizing,compatible-vendor-prefixes,gradients,vendor-prefix,import,zero-units,shorthand,important
 # The csslint rules which should cause the build to break if detected
 csslint.rules.break = parsing-errors