cmd/compile/internal/gc: cleanup walkselectcases slightly

Remove some unnecessary code. Most significantly, we can skip testing
"if ch == nil { block() }", because this is already the semantics
implied by normal send/receive operations.

Updates #40410.

Change-Id: I4acd33383cc876719fc3b998d85244d4ac1ff9d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/245126


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>