Fix misleading `:example_group` metadata deprecation warning.

Fixes #1562.
Because external libs can access metadata and wrongly try to access the `:example_group` key.  That's what VCR was doing, for example, before it got fixed in vcr/vcr#414.  We still want the warning when external things try to access the deprecated key.  We just don't want it when RSpec tries to due to a metadata filter previously configured.