Remove duplicate JSResourceReferenceImpl mock (#25976)

This mock exists in 2 directories (with identical implementation) and
Jest just picks one at random. This removes one which makes it at least
deterministic and fixes a Jest warning on startup.

It existed in these 2 places:
-
`packages/react-server-dom-relay/src/__mocks__/JSResourceReferenceImpl.js`
-
`packages/react-server-native-relay/src/__mocks__/JSResourceReferenceImpl.js`
(removed)

Author: kassens