glsl: Drop the ES requirement that VS outputs must be flat qualified.

glsl: Drop the ES requirement that VS outputs must be flat qualified.

Several conformance tests violate this requirement:

ES31-CTS.core.tessellation_shader.max_patch_vertices
ES31-CTS.core.tessellation_shader.tessellation_control_to_tessellation_evaluation.data_pass_through

I submitted a merge request to fix the conformance tests, but Khronos
opted to drop this GLSL ES specific requirement in favor of making flat
qualification of VS outputs optional, matching modern desktop GL.

Note that there were 7 Piglit tests which enforce this rule:
tests/spec/glsl-es-3.00/compiler/interpolation/qualifiers/*nonflat*
but these were deleted in Piglit commit acc0a2fabbd714bc704c16f1675e7c0.

Bugzilla: https://cvs.khronos.org/bugzilla/show_bug.cgi?id=15465#c7
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>