net: fs_enet: Remove fs_get_id()

fs_get_id() hasn't been used since commit b219108c ("fs_enet:
Remove !CONFIG_PPC_CPM_NEW_BINDING code")

Remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/7a53b88cc40302fcbea59554f5e7067e3594ad53.1691155346.git.christophe.leroy@csgroup.eu


Signed-off-by: Jakub Kicinski <kuba@kernel.org>