denc: remove conditional underlying_type_t from ceph_le* traits

denc: remove conditional underlying_type_t from ceph_le* traits

an enum's underlying type must be integral, so it's not possible for
ceph_le<T> to match for enums. for example, this:

  enum class enum_class_16 : ceph_le16 {};

fails to compile with:
underlying type ‘ceph_le16’ {aka ‘ceph_le<short unsigned int>’} of
‘enum_class_16’ must be an integral type

Signed-off-by: Casey Bodley <cbodley@redhat.com>