glsl: remove duplicate validation for index layout qualifier
glsl: remove duplicate validation for index layout qualifier

The minimum value for index is validated in apply_explicit_location()
and we want to remove validation from the parser so we can add
compile time constant support.

Reviewed-by: Samuel Iglesias Gonsálvez <b753b212f30aed827c3eea2e437609dc37310153@igalia.com>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler