barectf/templates: use `loop.last` instead of hack in `barectf/cgen.py`

barectf/templates: use `loop.last` instead of hack in `barectf/cgen.py`

This patch removes the raw text replacement at the end of
_CodeGen.gen_src() (`barectf/cgen.py`).

Instead, to avoid empty lines before `}`, Jinja 2 templates only
output an empty line it's not the loop's last iteration:

    {% if not loop.last %}{{ '\n' }}{% endif %}

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>