util: drop support for stack traces with logging

util: drop support for stack traces with logging

The log filters have supported the use of a "+" before the source match
string to request that a stack trace be emitted for every log message:

  commit 548563956e484e0e43e9a66a89bdda0f95930108
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed May 9 15:18:56 2012 +0100

    Allow stack traces to be included with log messages

    Sometimes it is useful to see the callpath for log messages.
    This change enhances the log filter syntax so that stack traces
    can be show by setting '1:+NAME' instead of '1:NAME'.

With the huge & ever increasing number of logging statements per file,
this will be incredibly verbose and have a major performance penalty.
This makes the feature impractical to use widely and as such it is not
worth the code maint cost.

Removing this seldom used feature allows us to drop the 'execinfo'
module in gnulib which provides the backtrace() function which doesn't
exist on non-Linux.

Users who want to get stack traces of parts of libvirt can use GDB,
or systemtap for live tracing with minimal perf impact.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>