net: dsa: dsa_legacy_fdb_{add,del} can be static

Introduced in commit 37b8da1a ("net: dsa: Move FDB add/del
implementation inside DSA") in net/dsa/legacy.c, these functions were
moved again to slave.c as part of commit 2a93c1a3 ("net: dsa: Allow
compiling out legacy support"), before actually deleting net/dsa/slave.c
in 93e86b3b

 ("net: dsa: Remove legacy probing support"). Along with
that movement there should have been a deletion of the prototypes from
dsa_priv.h, they are not useful.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210108233054.1222278-1-olteanv@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>