glsl: remove out of date comment
GLSL 4.40 changed the relevant language in Section 8.13.2 (Interpolation Functions) to:
"Component selection operators (e.g., .xy) may be used when specifying interpolant."
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GLSL 4.40 changed the relevant language in Section 8.13.2 (Interpolation Functions) to:
"Component selection operators (e.g., .xy) may be used when specifying interpolant."