Remove dead code umem.c

This removes the opensolaris umem.c defining memory allocation functions
which we do not use.

We should also remove umem.h from the same directory, but the messy
code makes this very difficult... It turns out that we have two
zfs_context.h versions, and one of them includes umem.h. If I try
to remove this version of zfs_context.h, compilation breaks. It
will take more effort to clean this mess up :(

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>