fix(docs): remove duplicate keys in wallet-rpc.md

[theborakompanioni]

Resolves #1552.

This is a hot fix to make the api docs served at https://joinmarket-org.github.io/joinmarket-clientserver/api/ work again.

Removes

'401':
  $ref: "#/components/responses/401-Unauthorized"

in favor of

'401':
  $ref: '#/components/responses/401-AuthenticationError'

when both are present.

This PR does not remove response 401-Unauthorized completely, as it is still used and I am not totally sure which one of them is actually returned.

How to test

cd docs/api
python3 -m http.server 9999

Then visit http://localhost:9999 and verify that the page loads as expected.

[kristapsk]

ACK af9f462

[kristapsk]

Merging, this seems like an obvious bugfix.