syntax-check: stop parsing .json files

These are ostensibly JSON, but what they actually are is Jinja
templates whose *output* is JSON. In particular, they can contain
{# comments in this syntax #}, causing syntax-check to fail.

lqa will check that their output is valid JSON anyway, so I think
the ability to have comments is worth more than the syntax check.

Reviewed-by: Luis Araujo <luis.araujo@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D2156