zcash_client_sqlite: Fix `select_spendable_notes` to not ignore the `sources` parameter

[str4d]

This is ignoring the _sources parameter to select_spendable_notes, and always selecting from both pools. It happens to cause no bugs because zcash_client_backend hard-codes both pools as sources if the feature flags are enabled, but we should fix this anyway.

Originally posted by @str4d in https://github.com/zcash/librustzcash/pull/1182#discussion_r1521502177