fix(AIP-203): remove optional conflict (#1182)

The AIPs now allow OPTIONAL to be used with other descriptive
annotations (e.g. INPUT_ONLY or IMMUTABLE). Removing the rule
accordingly to minimize confusion.

Author: toumorokoshi

docs/rules/0203/optional-conflict.md

@@ -36,7 +36,6 @@ func AddRules(r lint.RuleRegistry) error {
 		inputOnly,
 		immutable,
 		optional,
-		optionalBehaviorConflict,
 		outputOnly,
 		required,
 		unorderedListRepeated,