webrtcsrc: don't restrict RTP extensions to TWCC only

The motivation is the following: right now it is possible to add additional RTP extensions, apart from TWCC, with webrtcsink (with some hacks around request-encoded-filter, but possible).

However, even though these additional extensions are then received in an offer, they are dropped by webrtcsrc and the caps that are pushed out of its source pad contain only extmap for TWCC extension.

This happened because of an additional check in filter closure, which only allowed for TWCC extension, basically. The check was introduced by !1245 (merged).

This MR removes this check, but I wonder if there was a reason for this limitation.

If so, we may come with something more advanced, like conditionally enabling additional extensions based on a property value, adding whitelist property to let some extensions pass the filter, and so on.

cc @meh @thiblahute