tmem: cleanup: drop unused sub command

tmem: cleanup: drop unused sub command

TMEM_READ/TMEM_WRITE/TMEM_XCHG/TMEM_NEW_PAGE are never used, drop them to make
things simple and clean.

To be clear - we are bit lucky here - as none of the other implementors
of the tmem API are using it (Windows GPLPV code, SLES11, Linux upstream).

The spec says that the operations can return an error code (-ENOSYS for
example) so we are OK doing that.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>