committers-guide: Remove 'use-agent' from minimum gpg.conf settings

In the gpg2(1) man page 'use-agent' option is explained as following.

--use-agent
--no-use-agent
       This is dummy option. gpg2 always requires the agent.

It means adding 'use-agent' to gpg.conf is no-op. So remove it from
minimum acceptable defaults of gpg.conf.

Approved by: ygy (doc, mentor)