Revert "package/libsoxr: add avutil to soxr.pc"

This reverts commit d81870ae.

The patch attempts to fix static linking with libsoxr when it build with
avutils. The `Libs.private` field should not contain the full absolute path to
the static library, but only the link flags for private libraries, e.g
`-lm`.

Buildroots pkg-config prepends the sysroot to the value found in `Libs.private`
resulting in a malformed linker flag if libavutil is found:

```
-L/home/test/autobuild/run/instance-3/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib -lsoxr
/home/test/autobuild/run/instance-3/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/home/test/autobuild/run/instance-3/output/host/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libavutil.a
```

.. or if libavutils is not found:

```
-L/home/test/autobuild/run/instance-1/output/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib -lsoxr AVUTIL_LIBRARIES-NOTFOUND
```

Revert this commit and replace the patch by a follow-up patch which only
adds `-lavutil` to `Libs.private` in case it is found and used by
libsoxr.

Fixes:
http://autobuild.buildroot.net/results/6eb4e2c9bd3884ab0152ddf873c20e62f0941181/
http://autobuild.buildroot.net/results/07207b0a58a08bf7c2cb78345a58244b5e6aab0e/



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>