net: skbuff: remove special handling for SLOB

Commit c9929f0e

 ("mm/slob: remove CONFIG_SLOB") removes CONFIG_SLOB.
Now, we can also remove special handling for socket buffers with the SLOB
allocator. The code with HAVE_SKB_SMALL_HEAD_CACHE=1 is now the default
behavior for all allocators.

Remove an unnecessary distinction between SLOB and SLAB/SLUB allocator
after the SLOB allocator is gone.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230509071207.28942-1-lukas.bulwahn@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>