contrib: Remove knem source code

No longer supported in nemesis, remove knem source code.

contrib/knem/README

@@ -1,14 +0,0 @@
-knem ("kernel nemesis")
--------------------------
-This is a Linux kernel module that provides high-bandwidth message
-transfers on Linux when using the nemesis communication subsystem.  In
-order to use MPICH with knem you must build and install knem according
-configure MPICH with the following arguments:
-
-  --enable-device=ch3:nemesis(:net_mod)
-  --with-nemesis-local-lmt=knem
-  --with-knem-include=/path/to/knem/include/files
-
-Where (:net_mod) indicates an optional network module parameter such as
-tcp or mx.  Omitting a netmod will select the default netmod ("tcp" as
-of the time of this writing).