Remove duplicate definitions of Extract and Insert for UInt4

Change-Id: Ib2d54606ead6b953b93f28e9f0dfd7987b1da774
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28848
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>