net: dsa: tag_8021q: remove shim declarations

All users of tag_8021q select it in Kconfig, so shim functions are not
needed because it is not possible for it to be disabled and its callers
enabled.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>