Remove dead HTMLCanvasElement declaration in packages/flutter (#191165)

Clean up internal unreferenced `HTMLCanvasElement` JS interop
declaration in `packages/flutter/lib/src/web.dart` identified during
codebase reachability audit.

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- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
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- [x] I listed at least one issue that this PR fixes in the description
above.
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- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
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[Features we expect every widget to implement]:
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diff --git a/packages/flutter/lib/src/web.dart b/packages/flutter/lib/src/web.dart
index 230199d..76fc4d5 100644
--- a/packages/flutter/lib/src/web.dart
+++ b/packages/flutter/lib/src/web.dart
@@ -96,14 +96,6 @@
   external JSPromise<JSAny?> decode();
 }
 
-extension type HTMLCanvasElement._(JSObject _) implements HTMLElement, JSObject {
-  external int get width;
-  external set width(int value);
-  external int get height;
-  external set height(int value);
-  external String toDataURL();
-}
-
 extension type MediaQueryList._(JSObject _) implements EventTarget, JSObject {
   external bool get matches;
 }