cmd/compile/internal/syntax: remove code dealing with multiple method names

When parsing method declarations in an interface, the parser has
for historic reasons gracefully handled a list of method names with
a single (common) signature, and then reported an error. For example

        interface {
                m1, m2, m3 (x int)
        }

This code originally came from the very first parser for Go which
initially permitted such declarations (or at least assumed that
people would write such declarations). Nobody is doing this at this
point, so there's no need for being extra careful here. Remove the
respective code and adjust the corresponding test.

Change-Id: If6f9b398bbc9e425dcd4328a80d8bf77c37fe8b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/396654
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/syntax/parser.go

@@ -1725,20 +1725,6 @@ func (p *parser) methodDecl() *Field {
 	f.pos = p.pos()
 	name := p.name()
 
-	// accept potential name list but complain
-	// TODO(gri) We probably don't need this special check anymore.
-	//           Nobody writes this kind of code. It's from ancient
-	//           Go beginnings.
-	hasNameList := false
-	for p.got(_Comma) {
-		p.name()
-		hasNameList = true
-	}
-	if hasNameList {
-		p.syntaxError("name list not allowed in interface type")
-		// already progressed, no need to advance
-	}
-
 	const context = "interface method"
 
 	switch p.tok {

test/fixedbugs/bug121.go

@@ -9,7 +9,7 @@ package main
 type T func()
 
 type I interface {
-	f, g ();	// ERROR "name list not allowed"
+	f, g ();  // ERROR "unexpected comma"
 }
 
 type J interface {