MigrateSourcePluginManager::$classLoader property is unused

Added Patch

Ayup.

I confirmed that this is the only reference to this property in Migrate, and that it is never set:

[mandelbrot:maintainer | Fri 17:05:52] $ grep -rl "classLoader" * | grep -v "vendor"
core/lib/Drupal/Core/DrupalKernel.php
core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php
core/modules/simpletest/src/TestDiscovery.php
core/modules/simpletest/src/WebTestBase.php
core/tests/Drupal/KernelTests/KernelTestBase.php
core/tests/Drupal/Tests/BrowserTestBase.php
[mandelbrot:maintainer | Fri 17:05:57] $ grep -r "classLoader" core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php
core/modules/migrate/src/Plugin/MigrateSourcePluginManager.php:  protected $classLoader;

So we should indeed remove the property. This fix is also needed in 8.2.x. Since the property is protected, it is internal API, and since it is already removed without BC in 8.2.x, it does not make sense to retain the documentation of it in 8.2.x either.

Committed a772a40 and pushed to 8.3.x and 8.2.x. Thanks!