sRGB decode is controlled by format, not extra state. There is also no
565 sRGB format in Vulkan.

Also remove the 5 and 6bit sRGB LUTs since they are no longer used.

Change-Id: Iea8770a50e4ef77004dc66d4a9867df62e7c2aaf
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30368
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>