Remove the SYMFONY_DEPRECATIONS_HELPER environment variable

[amateescu]

• Since Drupal 11.3, core's own DeprecationHandler defaults to core/.deprecation-ignore.txt when SYMFONY_DEPRECATIONS_HELPER is unset, and on 11.x/main the same ignore file is configured directly in core/phpunit.xml.dist (overridable via DRUPAL_DEPRECATION_FILTER_CONFIG).
• The env var is only honored as a legacy override and resolves to the exact same effective configuration, so setting it in the DDEV web environment is redundant on every supported core branch.