Revert: Cache `BoxShadow` paints in `_BoxDecorationPainter` (#191346)

Reverts: [Cache `BoxShadow` paints in
`_BoxDecorationPainter`](https://github.com/flutter/flutter/pull/190194)

Initiated by: @cbracken

Reason for reverting:   ```

Original PR Author: @fa0311

Reviewed By: @victorsanni

The original PR description is provided below:

`_BoxDecorationPainter._paintShadows` called `BoxShadow.toPaint()` —
allocating a new `Paint` and native `MaskFilter` — for every shadow on
every paint, even though `BoxDecoration` is immutable and the painter is
retained across frames by `RenderDecoratedBox`. This PR caches the
paints on first use, the same way `_ShapeDecorationPainter` already
caches them in `_shadowPaints` (`shape_decoration.dart`).

The one observable difference matches existing `ShapeDecoration`
behavior: toggling `debugDisableShadows` between paints of a live
painter is not picked up until the painter is recreated, which is
debug-only and already the case for `ShapeDecoration`.

Part of #190187.

This is a behavior-preserving refactor; existing tests in
`packages/flutter/test/painting/` and
`packages/flutter/test/widgets/box_decoration_test.dart` cover this code
path and pass, so I believe this qualifies as test-exempt.

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we expect every widget to implement].
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Fixes] where supported.
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packages/flutter/lib/src/painting/box_decoration.dart

diff --git a/packages/flutter/lib/src/painting/box_decoration.dart b/packages/flutter/lib/src/painting/box_decoration.dart
index f113b43..a9ca00e 100644
--- a/packages/flutter/lib/src/painting/box_decoration.dart
+++ b/packages/flutter/lib/src/painting/box_decoration.dart
@@ -446,19 +446,12 @@
     }
   }
 
-  List<Paint>? _shadowPaints;
-
   void _paintShadows(Canvas canvas, Rect rect, TextDirection? textDirection) {
-    final List<BoxShadow>? boxShadows = _decoration.boxShadow;
-    if (boxShadows == null) {
+    if (_decoration.boxShadow == null) {
       return;
     }
-    final List<Paint> shadowPaints = _shadowPaints ??= <Paint>[
-      for (final BoxShadow boxShadow in boxShadows) boxShadow.toPaint(),
-    ];
-    for (var i = 0; i < boxShadows.length; i += 1) {
-      final BoxShadow boxShadow = boxShadows[i];
-      final Paint paint = shadowPaints[i];
+    for (final BoxShadow boxShadow in _decoration.boxShadow!) {
+      final Paint paint = boxShadow.toPaint();
       final Rect bounds = rect.shift(boxShadow.offset).inflate(boxShadow.spreadRadius);
       assert(() {
         if (debugDisableShadows && boxShadow.blurStyle == BlurStyle.outer) {