Revert "/api/schema.rs: implement Schema::Option"

This reverts commit 0a35462c1e.

I am not sure this add much value, and the old approach needs
less memory. If we really need single optional values, we can still
implement such Option while keeping the hash based approach...

src/getopts.rs

@@ -75,7 +75,7 @@ pub fn parse_arguments<T: AsRef<str>>(
                 None => {
                     let mut want_bool = false;
                     let mut can_default = false;
-                    if let Some(param_schema) = properties.get::<str>(&name) {
+                    if let Some((_optional, param_schema)) = properties.get::<str>(&name) {
                         if let Schema::Boolean(boolean_schema) = param_schema.as_ref() {
                             want_bool = true;
                             if let Some(default) = boolean_schema.default {