[wallet] [rpc] Remove getlabeladdress RPC

Summary:
labels are associated with addresses (rather than addresses being
associated with labels, as was the case with accounts). The
getlabeladdress does not make sense in this model, so remove it.

getaccountaddress is still supported for one release as the accounts
API is deprecated.

Backport of Core PR13060
https://github.com/bitcoin/bitcoin/pull/13060/

Depends on D4518

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4519