overrideDerivation: Simplify

Since we're just calling derivation again, drvAttrs is exactly what we
want to pass, and we can get rid of the complicated "drop" logic. As a
bonus, this actually makes it correct, since before "drop" removed the
attribute named passthru but not actually the attributes added to the
top-level attrset via passthru.

Signed-off-by: Shea Levy <shea@shealevy.com>