mail/mime-construct: Remove RUN_DEPENDS; reset maintainership

Remove converters/p5-MIME-Base64 from RUN_DEPENDS since it is already in
all supported Perl releases. While here, reset maintainership since it's
been 5 years since maintainer was last seen.

PR: 196700
Submitted by: Yasuhiro KIMURA
Approved by: maintainer timeout (4 months, maintainer reset)