cmd/compile: update fmt_test.go for esc.go code cleanup

CL 187598 removed a bunch of esc.go's debug messages, so some format
strings are no longer needed.

Fixes #33915.

Change-Id: Iaadf12b5d68358e1abb00210f5aa8be01b36ce82
Reviewed-on: https://go-review.googlesource.com/c/go/+/192142


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>