Revert "cmd/compile/internal/syntax: support for alias declarations"

This reverts commit 32db3f2756.

Reason: Decision to back out current alias implementation.

For #16339.

Change-Id: Ib05e3d96041d8347e49cae292f66bec791a1fdc8
Reviewed-on: https://go-review.googlesource.com/32825
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -229,7 +229,7 @@ redo:
 			goto assignop
 		}
 		if c == '-' {
-			s.tok = _Larrow
+			s.tok = _Arrow
 			break
 		}
 		s.ungetr()
@@ -253,16 +253,11 @@ redo:
 		s.tok = _Operator
 
 	case '=':
-		c = s.getr()
-		if c == '=' {
+		if s.getr() == '=' {
 			s.op, s.prec = Eql, precCmp
 			s.tok = _Operator
 			break
 		}
-		if c == '>' {
-			s.tok = _Rarrow
-			break
-		}
 		s.ungetr()
 		s.tok = _Assign