selinux: Delete mls_copy_context

This deletes mls_copy_context() in favor of mls_context_cpy() and
replaces mls_scopy_context() with mls_context_cpy_low().

Signed-off-by: Venkat Yekkirala <vyekkirala@TrustedCS.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>