bt2: don't copy swig interface files to build directory

A pet peeve of mine is that if you touch a bt2 py file (e.g. `touch
src/bindings/python/bt2/bt2/event.py`) and re-build, it will
re-run swig and rebuild the native module, even though it's not
necessary (all that is needed is to copy the .py files over to the build
directory).  This takes precious seconds from my development cycle.

This is done because the .i files are copied over to the build directory
(as they are in STATIC_BINDINGS_DEPS), along with the .py files.  And
since the modification time of the .i files has changed, it triggers the
bt2/native_bt.i -> bt2/native_bt.c conversion to run again.

Fix this by not copying the .i files to the build directory.  This is
not necessary anyway, as swig can read them perfectly fine from the
source directory.

And while at it, the logging.h/logging.c files don't need to be copied
to the build directory either, so make it so they are not copied and
read from the source directory (I don't think this provides any
advantage other than it being "cleaner").

Change-Id: Ib1ee03cd7795e0f9f3eaedb8476c052221acb9ee
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1712
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>