Remove old version of Linux if_pppol2tp.h

Remove old version of Linux if_pppol2tp.h

This has been in the Linux kernel source now for long enough that we
can rely on getting a usable version from /usr/include on all
distributions that we care about.  The version we have here had started
to lag behind what is in the Linux kernel, causing compilation errors
due to struct pppol2tpv3_addr being undefined.  Removing our local
version means we will use what is in /usr/include instead.

Signed-off-by: Paul Mackerras <paulus@samba.org>