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

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

commit 52a3c22210d3 ("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.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>