cmd/compile: remove ineffectual -i flag

This flag lost its usefulness in CL 34273.

Change-Id: I033c29f105937139b4e359a340906be439f1ed07
Reviewed-on: https://go-review.googlesource.com/c/141646
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -207,7 +207,6 @@ func Main(archInit func(*Arch)) {
 	objabi.Flagcount("e", "no limit on number of errors reported", &Debug['e'])
 	objabi.Flagcount("f", "debug stack frames", &Debug['f'])
 	objabi.Flagcount("h", "halt on error", &Debug['h'])
-	objabi.Flagcount("i", "debug line number stack", &Debug['i'])
 	objabi.Flagfn1("importmap", "add `definition` of the form source=actual to import map", addImportMap)
 	objabi.Flagfn1("importcfg", "read import configuration from `file`", readImportCfg)
 	flag.StringVar(&flag_installsuffix, "installsuffix", "", "set pkg directory `suffix`")