Dumps Celt in the Boneyard. Has officially been merged into the Opus
codec, which we already support, and has consequently ceased being
updated entirely at the current website and download link. See the
great big orange banner on http://www.celt-codec.org/

 :)

Closes Homebrew/homebrew#37051.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>