[identity] fix reserved username workflow for wallet users

Summary:
wallet users prove their identity with SIWE so there's no need for them to get a message from ashoat's keyserver.

one thing to note here is that users don't send OTKs when calling log_in_wallet_user, so the user won't have any OTKs initially. I think we made OTKs optional, though, so not sure how much this matters.

another thing to note is that if the user is registered here, their device list will be created by the identity service

Depends on D12515

Test Plan:
tested later in stack by registering a password and wallet user directly with my keyserver, which in turn called identity and added the address and username to the reserved usernames table

then, i flipped usingCSAT to true and tried logging in from web, which failed and displayed a message saying to log in from a mobile device

then, i tried logging in from native, which succeeded

then, i tried loggin in from web again, which succeeded

verified that username and wallet address were no longer in reserved usernames table

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12544