regexp: remove unreachable code

Found running go vet on the package. It barks that
	regexp/backtrack.go:257: unreachable code
	regexp/backtrack.go:302: unreachable code

For #11041

Change-Id: I0f5ba0d6183108fba3d144991b826273db0ffb09
Reviewed-on: https://go-review.googlesource.com/19824Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/backtrack.go

@@ -254,7 +254,6 @@ func (m *machine) tryBacktrack(b *bitState, i input, pc uint32, pos int) bool {
 
 			}
 			panic("bad arg in InstCapture")
-			continue
 
 		case syntax.InstEmptyWidth:
 			if syntax.EmptyOp(inst.Arg)&^i.context(pos) != 0 {
@@ -299,7 +298,6 @@ func (m *machine) tryBacktrack(b *bitState, i input, pc uint32, pos int) bool {
 			// Otherwise, continue on in hope of a longer match.
 			continue
 		}
-		panic("unreachable")
 	}
 
 	return m.matched