Don't require GStreamer

[uswoods]

Requiring GStreamer must be a joke.

error: failed to run custom build command for `gstreamer-webrtc-sys v0.7.0`
process didn't exit successfully: `/build/servo-git/src/servo/target/debug/build/gstreamer-webrtc-sys-0ea001806f557833/build-script-build` (exit code: 1)
--- stderr
`"pkg-config" "--libs" "--cflags" "gstreamer-webrtc-1.0 >= 1.14"` did not exit successfully: exit code: 1
--- stderr
Package gstreamer-webrtc-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gstreamer-webrtc-1.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'gstreamer-webrtc-1.0', required by 'virtual:world', not found

[ferjm]

Closing as a dup of #23050 which is asking for the same thing in a significantly more polite form.