internal/syscall/unix: simplify KernelVersion

internal/syscall/unix: simplify KernelVersion

Since values[2] elements are initialized with 0, the switch statement
doesn't do anything. Remove it.

Change-Id: I41176692cdf0c01fe8e85315f0c0dc8b0f3d41fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/427675
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>