cpp-common/bt2c/logging.hpp: remove no-formatting ("str") alternatives

cpp-common/bt2c/logging.hpp: remove no-formatting ("str") alternatives

This patch removes everything named bt2c::Logger::log*Str*() and
BT_CPPLOG*STR*().

The no-formatting versions existed to avoid a call to fmt::format_to()
when you need to log a literal string as is, without any formatting. It
was also useful to pass a user string as is.

However, in the end, I don't think such an optimization is necessary:

• We don't use the no-formatting versions that much compared to messages
  with a format string.

• fmt::format_to() is pretty fast without any replacement field.

• We're talking about logging performance.

• This patch removes a lot of often redudant code from `logging.hpp`.

Adapt the current no-formatting logging calls accordingly.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ie02ea492e220c7ce9b72aaf8728fb7d2211e0bc0
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12479
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>