Removed protections against user code redefining undefined

Jasmine hasn't even run on platforms that allowed redefining undefined
since 2.x.

src/core/matchers/DiffBuilder.js

@@ -31,10 +31,8 @@ getJasmineRequireObj().DiffBuilder = function(j$) {
 
         const actualCustom = this.prettyPrinter_.customFormat_(actual);
         const expectedCustom = this.prettyPrinter_.customFormat_(expected);
-        const useCustom = !(
-          j$.util.isUndefined(actualCustom) &&
-          j$.util.isUndefined(expectedCustom)
-        );
+        const useCustom =
+          actualCustom !== undefined || expectedCustom !== undefined;
 
         if (useCustom) {
           const prettyActual = actualCustom || this.prettyPrinter_(actual);

src/core/matchers/matchersUtil.js

@@ -177,13 +177,13 @@ getJasmineRequireObj().MatchersUtil = function(j$) {
       bStack,
       diffBuilder
     );
-    if (!j$.util.isUndefined(asymmetricResult)) {
+    if (asymmetricResult !== undefined) {
       return asymmetricResult;
     }
 
     for (const tester of this.customTesters_) {
       const customTesterResult = tester(a, b);
-      if (!j$.util.isUndefined(customTesterResult)) {
+      if (customTesterResult !== undefined) {
         if (!customTesterResult) {
           diffBuilder.recordMismatch();
         }