Remove distribution-specific init-scripts

Remove distribution-specific init-scripts

LSB-style (SysV-style) init scripts have not historically been
portable between distributions, as evidenced by the presence of both
"Red Hat" and "Slackware" init scripts in dbus. Many distributors
prefer to maintain them downstream, as is done in Debian (and its
derivatives) and in Slackware, so that the init script can follow
OS conventions (for example regarding boot messages) and make use
of OS-provided facilities (for example, the Debian init script uses
dpkg's start-stop-daemon utility).

The Slackware and Red Hat init scripts removed by this commit are not
tested or maintained in practice, and so are likely to have bugs. The
Slackware init-script provided here is not used on actual Slackware
systems, which provide a different implementation of rc.messagebus in
their packaging, while the Red Hat init script has been superseded by
the systemd unit in current Fedora, CentOS and RHEL versions.

The Cgywin messagebus-config provided here does appear to be used in
production in cygwin-ports, but it's full of Cygwin-specifics with which
the dbus maintainers are not familiar, so it is probably more appropriate
for it to be tracked downstream as part of the Cygwin packaging.

The systemd unit is not removed, since it is used on multiple Linux
distributions with little or no modification, and receives regular
testing and maintenance; this makes it appropriate to maintain upstream.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/101706
Reviewed-by: Philip Withnall <withnall@endlessm.com>