tests/plugins/flt.utils.trimmer/Makefile.am: remove useless HAVE_PYTHON guard

Philippe Proulx:
    The `test_trimming` test does not depend on Python: it uses the CLI,
    a `sink.text.details` component, and expectation strings to validate
    that an `flt.utils.trimmer` message iterator trims as expected.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If08b014259676a75c57db7eda109de55306aa062
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1727
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>