Documentation: Remove the deleted "framerelay" document from the index

commit f7365919 ("net: wan: Delete the DLCI / SDLA drivers")
deleted "Documentation/networking/framerelay.rst". However, it is still
referenced in "Documentation/networking/index.rst". We need to remove the
reference, too.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Link: https://lore.kernel.org/r/20201118124226.15588-1-xie.he.0141@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/index.rst

@@ -52,7 +52,6 @@ Contents:
    eql
    fib_trie
    filter
-   framerelay
    generic-hdlc
    generic_netlink
    gen_stats