radeonsi: remove special handling of TGSI_INTERPOLATE_COLOR in shader codegen
radeonsi: remove special handling of TGSI_INTERPOLATE_COLOR in shader codegen

It doesn't do anything useful. And colors are floating-point, so we can use
fs.interp, remove "flatshade" from the shader key, and rely on the FLAT_SHADE
state only (in the next patch).

Reviewed-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler