[RPC docs] Remove unused index files

Summary: These index files are not used and contain nothing useful. Now that D7130 and D7131 are landed, we know these index files will never be used.

Test Plan:

ninja doc-rpc

Verify doc/rpc/en/0.22.0/index.html and doc/rpc/en/0.22.0/rpc/index.html are no longer generated.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6596