I have found the builtin function in go src (/go/src/builtin/builtin.go), as following:
func delete(m map[Type]Type1, key Type)
But this is not source code. Who can tell me where is the builtin function source code?
builtin.go is simply a file for documentation purposes. It doesn't contain any implementations.
The map implementation is in runtime/hashmap.go. The delete functionality is implemented in the mapdelete function.