fix: remove this.link which results in mutating link chain

[Moniv9]

Fixes issue described here #8182.

Due to single instance of apolloClient in SPA, this.link keeps on adding/preserving new/old apollo-links, which all runs on graphQL request/response.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests

[apollo-cla]

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[Moniv9]

@benjamn @hwillson if you can review this change.

[hwillson]

Thanks for this @Moniv9 and sorry for the delay, but this is by design. We're intentionally allowing the link chain to be mutated so people can adjust links after ApolloClient instantiation. We need the updated link chain to be stored in the link instance variable so it can be effectively used by other parts of the ApolloClient class that run outside of the QueryManager.