uapi: remove telephony headers

ixjuser.h includes the telephony.h header. Other than that no kernel
code uses any of these headers. The last user of the ixjuser.h header
has been removed in commit 7326446c

 (Staging: remove telephony
drivers), more than 5 years ago.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>