Fix memo preflight eligibility by reverting gas-asset-only assumption from MR !4581

Summary

This MR corrects memoless preflight eligibility in /thorchain/memo/check/{asset}/{amount}.

Context

MR !4581 (merged) (from issue #2776 (closed)) introduced a gas-asset-only restriction in memo preflight. That change was based on an incorrect assumption: non-gas assets (ERC20 tokens) are not memoless-eligible.

The actual protocol behavior is:

• direct token transfer to vault is not supported
• router-based token deposit (approve + depositWithExpiry) is supported
• therefore, non-gas assets should not be rejected by memo preflight

What changed

• Removed the !asset.IsGasAsset() rejection from queryReferenceMemoPreflight.
• Kept memoless state checks:
  • HaltMemoless
  • MemolessTxnTTL > 0

Tests

• Updated querier tests to reflect correct eligibility behavior:
  • non-gas ERC20 asset with pool is preflight-eligible
• make test passes
• make lint passes
• make test-simulation passes

References

• Incorrect assumption introduced in MR !4581 (merged)
• Related issue: #2776 (closed)
• Prior clarification context: #2773 (closed)