build: Remove unused options.
* configure.ac: Remove option --build-agent-only. (FAKE_CURL, GPGKEYS_CURL): Remove check for cURL (GPGKEYS_MAILTO): Remove ac_subst but keep the currently unused SENDMAIL check. (GPGKEYS_KDNS): Remove ac_subst. * autogen.rc (final_info): Remove suggestion to use the removed option --enable-mailto.