glsl: do not set the 'smooth' qualifier by default on ES shaders

glsl: do not set the 'smooth' qualifier by default on ES shaders

It leads to surprising states with integer inputs and outputs on
vertex processing stages (e.g. geometry stages). Instead, rely on the
driver to choose smooth interpolation by default.

We still allow varyings to match when one stage declares it as smooth
and the other declares it without interpolation qualifiers.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>