cmd/compile: remove dead code handling '~' operator

The parser code was not reachable ever since some of the lexer cleanups.
We could recognize '~' in the lexer, complain, and return a '^' instead,
but it's been a few years since Go was new and this may have been a use-
ful error. The lexer complains with "illegal character U+007E '~'" which
is good enough.

For #13244.

Change-Id: Ie3283738486eb6f8462d594f2728ac98333c0520
Reviewed-on: https://go-review.googlesource.com/20768Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -1161,13 +1161,6 @@ func (p *parser) uexpr() *Node {
 	case '!':
 		op = ONOT
 
-	case '~':
-		// TODO(gri) do this in the lexer instead (issue 13244)
-		p.next()
-		x := p.uexpr()
-		Yyerror("the bitwise complement operator is ^")
-		return Nod(OCOM, x, nil)
-
 	case '^':
 		op = OCOM
 
@@ -2517,7 +2510,7 @@ func (p *parser) stmt() *Node {
 
 	case LNAME, '@', '?', LLITERAL, LFUNC, '(', // operands
 		'[', LSTRUCT, LMAP, LCHAN, LINTERFACE, // composite types
-		'+', '-', '*', '&', '^', '~', LCOMM, '!': // unary operators
+		'+', '-', '*', '&', '^', LCOMM, '!': // unary operators
 		return p.simple_stmt(true, false)
 
 	case LFOR: