anv/pipeline: Only call remove_dead_variables once
anv/pipeline: Only call remove_dead_variables once

It can handle multiple modes at a time now so there's no reason to call
it repeatedly.

Reviewed-by: Timothy Arceri <5dd83789ef3714029244367ba1ca09f1d7627408@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler