Remove separate agl-{cluster,telematics}-demo features

Remove separate agl-{cluster,telematics}-demo features

Changes:
- Remove feature templates and associated include files for the
  agl-cluster-demo and agl-telematics-demo features.  All of the
  demo images can now be built by just configuring with the agl-demo
  feature.
- Remove feature template for agl-cluster-demo-preload, the cluster
  demo configuration it was pulling into the cluster image is now
  driven by the agl-demo-preload feature instead.
- Moved speech framework and Alexa voiceagent feature inclusion to
  agl-demo-platform.inc to avoid them accidentally being pulled into
  the cluster or telematics demo images.  Part of this change is
  moving agl-voiceagent-alexa from using IMAGE_FEATURES to
  AGL_FEATURES, which needs a corresponding change to meta-agl-devel.
- Remove leftover use of agl-core-image-profile DISTRO_FEATURE, which
  no longer exists.
- Remove unneeded explicit dependency on the
  PREFERRED_RPROVIDER_virtual/voice-high-config, things seem to be
  working fine with the dependency from agl-service-voice-high, so it
  is not required.

Bug-AGL: SPEC-3749

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ibcfbce28231a91aa56269db9196c471458d0e928