Remove subscription section from README (graphql#4487)

Removed subscription section from README as that directory is no longer
present.

Author: NeoPhi

src/README.md

@@ -20,4 +20,3 @@ Each sub directory within is a sub-module of graphql-js:
 - [`graphql/error`](error/README.md): Creating and formatting GraphQL errors.
 - [`graphql/utilities`](utilities/README.md): Common useful computations upon
   the GraphQL language and type objects.
-- [`graphql/subscription`](subscription/README.md): Subscribe to data updates.