Revert "librtmp: Set rtmp tarball to gz, since that is what is actually downloaded."
Revert "librtmp: Set rtmp tarball to gz, since that is what is actually downloaded."

This reverts commit 6a939429f1ac7c9453a2287bdc792fa29aba5a5c.

We can just fix the tarball instead, we host it after all.

https://bugzilla.gnome.org/show_bug.cgi?id=774796

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