kbuild: deb-pkg: remove support for "name <email>" form for DEBEMAIL

kbuild: deb-pkg: remove support for "name <email>" form for DEBEMAIL

Commit d5940c60e057 ("kbuild: deb-pkg improve maintainer address
generation") supported the "name <email>" form for DEBEMAIL, with
behavior slightly different from devscripts.

In Kbuild, if DEBEMAIL is given in the form "name <email>", it is used
as-is, and DEBFULLNAME is ignored.

In contrast, debchange takes the name from DEBFULLNAME (or NAME) if set,
as described in 'man debchange':

  If this variable has the form "name <email>", then the maintainer name
  will also be taken from here if neither DEBFULLNAME nor NAME is set.

This commit removes support for the "name <email> form for DEBEMAIL,
as the current behavior is already different from debchange, and the
Debian manual suggests setting the email address and name separately in
DEBEMAIL and DEBFULLNAME. [1]

If there are any complaints about this removal, we can re-add it,
with better alignment with the debchange implementation. [2]

[1]: https://www.debian.org/doc/manuals/debmake-doc/ch03.en.html#email-setup
[2]: https://salsa.debian.org/debian/devscripts/-/blob/v2.23.7/scripts/debchange.pl#L802

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>