anv/pipeline: Stop claiming to support running without a vertex shader

anv/pipeline: Stop claiming to support running without a vertex shader

From the Vulkan spec version 1.0.32 docs for vkCreateGraphicsPipelines:

    The stage member of one element of pStages must be
    VK_SHADER_STAGE_VERTEX_BIT

Since a vertex shader is always required, this hasn't been used since we
deleted meta.  Let's get rid of the complexity.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>