Delete release_2.19.4.0 branch on prometheus exporter plugin repo

[lukas-vlcek]

Please, delete the release_2.19.4.0 branch. It is a leftover from old release and I can not delete it manually.

https://github.com/opensearch-project/opensearch-prometheus-exporter/branches

Thank you.

[peterzhuamazon]

Can one of @KarstenSchnitter @ps48 @ritvibhatt co-approve this?

Thanks.

[KarstenSchnitter]

Please go on with the deletion.

[peterzhuamazon]

Removed release_2.19.4.0 as requested here. Thanks.