Always re-generate the manpage for dh_bash-completion

The manpage for dh_bash-completion is currently not re-generated during
builds, because it is not listed as a dependency of any debian/rules
target.  Packaging only works because the output file is present in the
repository.

This patch removes the output file from the repository and adds it as a
dependency of the dh_install target, so that the manpage gets
re-generated during builds.

This patch also replaces the manual parsing of the upstream version with
the DEB_VERSION_UPSTREAM from /usr/share/dpkg/pkg-info.mk.  Thanks to
Juhani Numminen for suggesting this change [1].

[1] https://bugs.debian.org/877450#81