Enable the universe proof courier by default for all itests.

[ffranr]

The universe RPC proof courier should be enabled by default for all integration tests. We should also retain the ability to specify the use of the hashmail proof courier for certain itests.

Completing this work should allow use to simplify our code by getting rid of checks such as:

taproot-assets/tapgarden/custodian.go

Line 375 in 1e24b99

if c.cfg.ProofCourierCfg == nil {

And allow us to remove this helper function which we currently use to sidestep the proof transfer send step:

taproot-assets/itest/addrs_test.go

Line 279 in 90b0ac3

func sendProof(t *harnessTest, src, dst *tapdHarness, scriptKey []byte,

This helper function was created before we had a reliable proof courier.

[ffranr]

There are 28 sendProof call sites.