[Cashtab] [Remove BCH param from wallet derivation p1] Remove use of slpAddress from Cashtab

Summary:
T2836

Remove use of legacy address types from Cashtab as prep for removing them from wallet object

Currently, only wallet.Path1899.slpAddress is used anywhere in the app. Remove it.

Test Plan:
npm start
Send an airdrop with 'Ignore my own address' checked
Confirm it ignores your address
grep -r wallet.Path1899.slpAddress src/ and no result
grep -r wallet.Path1899.legacyAddress src/ and no result
grep -r wallet.Path1899.fundingAddress src/ and no result

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12749