Remove legacy printf_verbose()/printf_debug() and others

This patch removes all the legacy logging macros found in
babeltrace-internal.h. It also removes the babeltrace_verbose and
babeltrace_debug symbols.

To remain as backward compatible as possible, the CLI checks the
BABELTRACE_VERBOSE and BABELTRACE_DEBUG environment variables and
overrides the default log level (WARN, or the one set with --log-level)
with VERBOSE if BABELTRACE_DEBUG is 1 and INFO if only
BABELTRACE_VERBOSE is 1. This default log level applies to all the known
loggers, except the ones that have their log level explicitly set with a
dedicated environment variable.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>