Formally deprecated celt 0.5.1 usage

Quoting git log from spice-common 72b0d60 "build: Disable celt 0.5.1
by default" in 2018-06-05 [0]

  |  build: Disable celt 0.5.1 by default
  |
  |  This version of the CELT codec has long been obsolete, and
  |  Opus support has been added nearly 5 years ago. It's time
  |  we move on and try to stop using Celt ;)
  |  This commit disables CELT by default, but since
  |  this could be an unexpected change for packagers, if CELT
  |  0.5.1 development headers are installed, it will error
  |  out unless --enable-celt051/--disable-celt051 has
  |  been explicitly specified.

[0] spice-common@72b0d603



This change is part of spice server since f5785db1 or v0.14.1 and
spice-gtk e6e9d7a or v0.35.

It is time to formally deprecated it and remove from all  components
in favor of opus mode.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>