package/celt051: drop package

package/celt051: drop package

Remove celt051 package as celt has been merged into the IETF Opus codec
and is now obsolete (see http://celt-codec.org/).

The only reverse dependency of celt051 is spice. Opus support on spice
has been added upstream 6 years ago with:
https://gitlab.freedesktop.org/spice/spice/commit/ce9b714137a767b81f2d3c40b5f3ce0d5cf70fc8

Spice disabled celt by default since version 0.14.1 and:
https://gitlab.freedesktop.org/spice/spice-common/commit/72b0d603e128cd3af15974fe6b3e4b56ea9c6f34

Spice evens error out, by default, if Opus is missing but not explicitly
disabeld since:
https://gitlab.freedesktop.org/spice/spice-common/commit/f5224738425a7e97e1c32ea85bb3b243e50d4e22

This will also fix a static build failure on spice with celt051 and opus.

Fixes:
 - http://autobuild.buildroot.org/results/96c786f85d35f33508e9c71778043d16b87f72cd

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: slight rephrasing in legacy help]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>