Speed up FirewallController startup.

FirewallController::createChain runs iptables commands to remove
the newly-created chain from fw_INPUT. This is not necessary,
because createChain is only called from setupIptablesHooks, which
is only called immediately after initIptablesRules, which clears
fw_INPUT. So there is nothing to delete.

Removing these unnecessary commands speeds up netd startup by
~150ms.  Before:
02-03 18:51:40.075   492   492 I Netd    : Setting up FirewallController hooks: 159.9ms

After:
02-03 18:45:22.005   489   489 I Netd    : Setting up FirewallController hooks: 11.3ms

Bug: 34873832
Test: unit tests continue to pass
Change-Id: I651d96a71c98d6aba989927cd23036d5cc371dd7