Fix comments with continuation

Backgound: aog/919954 tried to handle
```
   second line
```

but did it incorrectly. The parser works correctly (so this change
reverts aog/919954), it returns multiline comment, but the serializer
converting the internal representation to Blueprint was not emitting
'//' on the lines after the first.

Test: treehugger
Bug: 127521510
Change-Id: I0257a8b3cc4ffcaa6bea44113ceba66bb99d7e43

androidmk/parser/parser.go

@@ -497,12 +497,6 @@ loop:
 		switch p.tok {
 		case '\\':
 			p.parseEscape()
-			if p.tok == '\n' {
-				// Special case: '\' does not "escape" newline in comment (b/127521510)
-				comment += "\\"
-				p.accept(p.tok)
-				break loop
-			}
 			comment += "\\" + p.scanner.TokenText()
 			p.accept(p.tok)
 		case '\n':