Remove now-redundant NestedMatcher-based code.

core/lib/Drupal/Core/CoreBundle.php

core/lib/Drupal/Core/Routing/ChainMatcher.php

core/lib/Drupal/Core/Routing/FirstEntryFinalMatcher.php

core/lib/Drupal/Core/Routing/HttpMethodMatcher.php

core/lib/Drupal/Core/Routing/InitialMatcherInterface.php

core/lib/Drupal/Core/Routing/NestedMatcher.php

core/lib/Drupal/Core/Routing/NestedMatcherInterface.php

core/lib/Drupal/Core/Routing/PathMatcher.php

...stem/lib/Drupal/system/Tests/Routing/ChainMatcherTest.php

...rupal/system/Tests/Routing/FirstEntryFinalMatcherTest.php

...lib/Drupal/system/Tests/Routing/HttpMethodMatcherTest.php

...m/lib/Drupal/system/Tests/Routing/MimeTypeMatcherTest.php

...ystem/lib/Drupal/system/Tests/Routing/MockPathMatcher.php

...tem/lib/Drupal/system/Tests/Routing/NestedMatcherTest.php

...ystem/lib/Drupal/system/Tests/Routing/PathMatcherTest.php