Remove modified_files delegator from PostCheckout::Base
Remove modified_files delegator from PostCheckout::Base

This was done in error in 8ee8707, since `Hook::Base` already defines a
delegator for `modified_files`.

gnagel/overcommit,gnagel/overcommit,blech75/overcommit,blech75/overcommit,oxocode/overcommit,oxocode/overcommit,TheSooth/overcommit,TheSooth/overcommit