tiff: remove InternalError which is not used anywhere.

Change-Id: I92fd800bafd380797651e158c9385dcf3d3578da
Reviewed-on: https://go-review.googlesource.com/40892
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

 tiff/reader.go | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/tiff/reader.go b/tiff/reader.go
index cfe6fed..0e3d332 100644
--- a/tiff/reader.go
+++ b/tiff/reader.go
@@ -35,13 +35,6 @@ func (e UnsupportedError) Error() string {
 	return "tiff: unsupported feature: " + string(e)
 }
 
-// An InternalError reports that an internal error was encountered.
-type InternalError string
-
-func (e InternalError) Error() string {
-	return "tiff: internal error: " + string(e)
-}
-
 var errNoPixels = FormatError("not enough pixel data")
 
 type decoder struct {