package/alsa-utils: Fix alsa-plugins compatibility

Previously, alsa-plugins would not work if alsa-utils was installed
after it. This happened because:

1. alsa-plugins copies some files $(TARGET_DIR)/usr/share/alsa/alsa.conf.d
2. alsa-utils removes these files during installation ( rm -rf $(TARGET_DIR)/usr/share/alsa/;)

The `rm -rf` command was originally added as part of the fix for
https://bugs.buildroot.org/show_bug.cgi?id=1573

 11 years ago.

The intention might have been to allow for unconfiguring some options
and then rebuilding alsa-utils. However, this is a scenario that does
not work anyway.

The simplest fix for the `alsa-plugins` compatibility issue appears to
be to remove the `rm -rf` command.

Signed-off-by: Gleb Mazovetskiy <glex.spb@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>