go/printer: simply ignore filename changes in position information

There's no good reason to make any printer state adjustments
simply because the file name in node position information has
changed. Eliminate the relevant code.

R=r
CC=golang-dev
https://golang.org/cl/6856054

src/pkg/go/printer/printer.go

@@ -221,21 +221,6 @@ func (p *printer) writeString(pos token.Position, s string, isLit bool) {
 		// atLineBegin updates p.pos if there's indentation, but p.pos
 		// is the position of s.
 		p.pos = pos
-		// reset state if the file changed
-		// (used when printing merged ASTs of different files
-		// e.g., the result of ast.MergePackageFiles)
-		if p.last.IsValid() && p.last.Filename != pos.Filename {
-			// Note: Do not set p.indent to 0 - this seems to be a bad heuristic.
-			//       ASTs may be created by various tools and built from nodes of
-			//       different files. An incorrectly constructed AST will likely
-			//       not print at all, but a (structurally) correct AST with bad
-			//       position information should still print structurally correct.
-			//       If p.indent is reset, indentation may be off, and likely lead
-			//       to indentation underflow (to detect set: debug = true).
-			//       See also issue 4300 (11/16/2012).
-			p.mode = 0
-			p.wsbuf = p.wsbuf[0:0]
-		}
 	}
 
 	if isLit {