kbuild: deb-pkg: remove support for EMAIL environment variable

Commit edec611d ("kbuild, deb-pkg: improve maintainer
identification") added the EMAIL and NAME environment variables.

Commit d5940c60 ("kbuild: deb-pkg improve maintainer address
generation") removed support for NAME, but kept support for EMAIL.

The EMAIL and NAME environment variables are supported by some tools
(see 'man debchange'), but not by all.

We should support both of them, or neither of them. We should not stop
halfway.

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