schema/schema.json: Make 'hostname' optional

The JSON Schema requirement dates back to cdcabde (schema: JSON
Schema and validator for `config.json`, 2016-01-19, opencontainers#313), but the
property has been explicitly optional in the Markdown spec since
7ac41c6 (config.md: reformat into a standard style, 2015-06-30).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

schema/schema.json

@@ -177,7 +177,6 @@
         "platform",
         "process",
         "root",
-        "hostname",
         "mounts",
         "hooks"
     ]