asterisk-15.x: remove sounds cache

asterisk-15.x: remove sounds cache

When there is no sounds cache declared the build system will not attempt to
download any sound packs or their SHA1 checksums.

This is to be preferred because:

  a) the build may occur offline, causing it to fail
  b) plain http is used by the build system for downloading

There is no drawback here because the standard sound packs are included in
the Asterisk source tarball already.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>