This reverts commit 8226c94110.
Now that GStreamer 1.20 is out, the v1_20 feature can and should link
directly to the stable release instead of the unstable 1.19 development
release.
The `links` annotation in `Cargo.toml` is intended to ensure that in the
crate graph there's at most one crate that's an implementation of
some sort concept.
This can make sense in some scenarios, most prominent of which is when
the crate defines `#[no_mangle]` symbols (e.g. by compiling a vendored C
library.) In that situation linking a binary that depends on two
versions of the library cannot work because of colliding symbol names.
There does not appear to be a similar reason to impose such a
restriction on the users of `gstreamer-sys` and similar, however. All of
these crates link to a system library, they do not define any
`#[no_mangle]` symbols nor they vendor and build C libraries as part of
their build process. All they do is linking to a system library. Most
likely all the different versions of the bindings will link to the exact
same library too.
I haven't seen any global resources that these bindings use to ensure
soundness of the library, either.
