stdenv: Revert special-casing of fixupPhase

Treating fixupPhase specially is really ugly.  Also, it collides with
the work in the multiple-outputs branch (which already has support for
fixing up all outputs).

Partial revert of 0a44a091217ecd335cd10c06c1ddd29ad599f18c.
stdenv: Update comment
stdenv: Fix indentation