nixos/chrony: remove redundant 'initstepslew.servers' option

This option was added in 6336048c

 but it
is essentially a complete duplicate of the existing cfg.servers and
there seems to be no reason to keep maintaining it.

Furthermore, it requires annoying duplication if you try to do option
merging, e.g. merging in sets into your configuration.nix that add
`services.chrony.initstepslew` options will overwrite the servers option
unless you keep it, but that means you just have to duplicate
config.networking.timeServers again anyway which is an implementation
detail!

Signed-off-by: Austin Seipp <aseipp@pobox.com>