clang: remove pptp[d].c from the build

If somebody wants to revive this turkey then make it compile cleanly at first.

But first, find someone who cares.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

sys/src/cmd/ip/ip.json

@@ -20,8 +20,6 @@
 			"httpfile.c",
 			"linklocal.c",
 			"pppoe.c",
-			"pptp.c",
-			"pptpd.c",
 			"rarpd.c",
 			"rexexec.c",
 			"rip.c",