cmd/compile/internal/gc: remove unused methodbyname type

cmd/compile/internal/gc: remove unused methodbyname type

methodbyname was used for sorting in bexport.go, until
https://golang.org/cl/139338 removed the code that
invoked sorting function.

R=1.13

Change-Id: I13e313fb60111a142ed3883d81916af254445fdc
Reviewed-on: https://go-review.googlesource.com/c/155959
Run-TryBot: Iskander Sharipov <quasilyte@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>