cmd/compile: eliminate "assignment count mismatch" - not needed anymore

See https://go-review.googlesource.com/#/c/38313/ for background.
It turns out that only a few tests checked for this.

The new error message is shorter and very clear.

Change-Id: I8ab4ad59fb023c8b54806339adc23aefd7dc7b07
Reviewed-on: https://go-review.googlesource.com/38314
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3436,7 +3436,7 @@ func typecheckas2(n *Node) {
 	}
 
 mismatch:
-	yyerror("assignment count mismatch: cannot assign %d values to %d variables", cr, cl)
+	yyerror("cannot assign %d values to %d variables", cr, cl)
 
 	// second half of dance
 out:

test/fixedbugs/bug289.go

@@ -9,14 +9,14 @@
 package main
 
 func f1() {
-	a, b := f()	// ERROR "mismatch|does not match"
+	a, b := f()	// ERROR "cannot assign|does not match"
 	_ = a
 	_ = b
 }
 
 func f2() {
 	var a, b int
-	a, b = f()	// ERROR "mismatch|does not match"
+	a, b = f()	// ERROR "cannot assign|does not match"
 	_ = a
 	_ = b
 }

test/fixedbugs/bug487.go

@@ -14,8 +14,8 @@ func G() (int, int, int) {
 }
 
 func F() {
-	a, b := G()	// ERROR "mismatch"
-	a, b = G()	// ERROR "mismatch"
+	a, b := G()	// ERROR "cannot assign"
+	a, b = G()	// ERROR "cannot assign"
 	_, _ = a, b
 }