Remove kWebKitErrorCannotShowMimeType as unused.

Bug: 546191
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I870931443b05f0ea8767864346703ba954df63f0
Reviewed-on: https://chromium-review.googlesource.com/c/1287291Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600623}

ios/web/public/web_kit_constants.h

@@ -13,9 +13,6 @@ namespace web {
 // Indicates WebKit errors.
 extern const char kWebKitErrorDomain[];
 
-// MIME type is not supported.
-const long kWebKitErrorCannotShowMimeType = 100;
-
 // Can not change location URL.
 const long kWebKitErrorCannotShowUrl = 101;