Drop unnecessary Hasher::finish() in AssetBase::hash

[dmidem]

This PR removes the h.finish(); line in src/note/asset_base.rs (AssetBase::hash) because it does nothing useful there and it generates the unused_must_use compilation warning when compiling Orchard as a librustzcash dependency:

Checking orchard v0.11.0 (/home/dima/devel/qedit/zsa/orchard)
warning: unused return value of `finish` that must be used
   --> /home/dima/devel/qedit/zsa/orchard/src/note/asset_base.rs:149:9
    |
149 |         h.finish();
    |         ^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
    |
149 |         let _ = h.finish();
    |         +++++++

warning: `orchard` (lib) generated 1 warning