freedreno/ir3: remove half-precision output

freedreno/ir3: remove half-precision output

This doesn't really work, we can't necessarily just change the outputs
to half-precision like this in anything but simple cases.

Keep the shader key entry around though, eventually with proper mediump
support we could use this with a nir pass to use lower precision frag
shader outputs when the render target format has <= 16b/component.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Eric Anholt <eric@anholt.net>