Revert "Fix: Add blend mode fallbacks if not supported (#758)" (#769)

This reverts commit 95def3b.

src/lib/polyfill.js

@@ -392,28 +392,4 @@ if (!String.prototype.startsWith) {
         return this.substr(!pos || pos < 0 ? 0 : +pos, search.length) === search;
     };
 }
-
-// Provides a partial workaround when blend modes are not supported (IE11).
-// See issue https://github.com/mozilla/pdf.js/issues/5264.
-// NOTE: This is not a full blend mode polyfill, but a way to see blended content
-// that would otherwise be unviewable.way to see blended content that would otherwise be unviewable.
-const canvas = document.createElement('canvas');
-const ctx = canvas.getContext('2d');
-ctx.globalCompositeOperation = 'multiply';
-
-// Check to see if the current browser supports globalCompositeOperation, which would be the case if the
-// set value is respected.
-if (ctx.globalCompositeOperation !== 'multiply') {
-    // Define custom behavior for blending when blend modes are not supported.
-    Object.defineProperty(CanvasRenderingContext2D.prototype, 'globalCompositeOperation', {
-        set: function(value) {
-            // We only manually blend if the mode is not 'source-over'
-            // which is the only supported blend mode that does no blending.
-            if (value !== 'source-over' && this.globalAlpha === 1) {
-                // Choose 50% to give equal visibility to top and bottom elements.
-                this.globalAlpha = 0.5;
-            }
-        }
-    });
-}
 /* eslint-enable */