MINOR: proxy: add free_logformat_list() helper function

MINOR: proxy: add free_logformat_list() helper function

There are multiple places inside free_proxy() where we need to perform
the exact same operation: freeing a logformat list which includes freeing
every member.

To prevent code duplication, we add the free_logformat_list() function
that takes such list as parameter and does all the freeing job on its
own.