Revert "cmd/compile: improve error message if init is directly invoked"

This reverts commit cb6e0639fb.

The fix is incorrect as it's perfectly fine to refer to an
identifier 'init' inside a function, and 'init' may even be
a variable of function value. Misspelling 'init' in that
context would lead to an incorrect error message.

Reopened #8481.

Change-Id: I49787fdf7738213370ae6f0cab54013e9e3394a8
Reviewed-on: https://go-review.googlesource.com/37876
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/init.go

@@ -15,7 +15,7 @@ func init() {
 }
 
 func main() {
-	init()         // ERROR "cannot refer to init functions"
+	init()         // ERROR "undefined.*init"
 	runtime.init() // ERROR "unexported.*runtime\.init"
-	var _ = init   // ERROR "cannot refer to init functions"
+	var _ = init   // ERROR "undefined.*init"
 }