[ns sym] -> nil
Removes a mapping for a symbol from the target namespace.
Namespace mappings represent top level bindings created by clj::clojure.core/def, and aliases for defs.
Note that mappings are distinct from aliases in that where mappings represent defs or renamings of defs (see clj::clojure.core/refer
:rename) where aliases allow shorthand for other namespaces (see clj::clojure.core/require