Remove unused variable $next from aggregator module

I have checked that the variable is not used anywhere in the class AggregatorController.
The variable can safely be removed.
For me it is RTBC.

Thank you for your work on cleaning up Drupal core's code style!

In order to fix core coding standards in a maintainable way, all our coding standards issues should be done on a per-rule basis across all of core, rather than fixing standards in individual modules or files. We should also separate fixes where we need to write new documentation from fixes where we need to correct existing standards. This all should be done as part of #2571965: [meta] Fix PHP coding standards in core, stage 1. For issues around unused variables please see #3106216-11: Remove unused variables from core.

For background information on why we usually will not commit coding standards fixes that aren't scoped in that way, see the core issue scope guidelines, especially the note about coding standards cleanups. That document also includes numerous suggestions for scoping issues including documentation coding standards cleanups.

Contributing to the overall plan above will help ensure that your fixes for core's coding standards remain in core the long term.