busybox: remove actions that depend on removed netkit* packages

Both "netkitbase" and "netkittelnet"  packages were removed by this
commit:
http://git.buildroot.net/buildroot/commit/?id=560fe85bf7fb81956e80848c8bb28d7a6d00a2a2



Now we may safely remove reminders in busybox.mk that depend on
mentioned packages that are no longer in Buildroot.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>