This reverts commit b6099d09ff.
The endpoint existed so an external installer could drop a tenant's provider
cache after installing plugins behind this service's back. Disabling the
cache outright removes the need: there is nothing to invalidate, and the
endpoint has no caller.
PluginService.invalidate_plugin_model_providers_cache stays. It predates the
endpoint and is still called from this service's own plugin lifecycle paths.