Surplus code related to the basevertex is removed.

The Vertex Elements contain now:
* VE 1: <firstvertex, BaseInstance, VertexID, InstanceID>
* VE 2: <DrawID, is_indexed_draw, 0, 0>

Also fixes unreachable message.

Fixes OpenGL CTS tests:
* KHR-GL46.shader_draw_parameters_tests.ShaderDrawArraysInstancedParameters
* KHR-GL46.shader_draw_parameters_tests.ShaderMultiDrawArraysParameters
* KHR-GL46.shader_draw_parameters_tests.MultiDrawArraysIndirectCountParameters
* KHR-GL46.shader_draw_parameters_tests.ShaderDrawArraysParameters
* KHR-GL46.shader_draw_parameters_tests.ShaderMultiDrawArraysIndirectParameters

Fixes Piglit tests:
* arb_shader_draw_parameters-drawid-indirect baseinstance
* arb_shader_draw_parameters-basevertex

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102678