Skip jinja2 macros to build filter SSR expressions

Our custom Markdown tag `[filter]` currently relies on one unnecessary transpiling step. The flow is as follows:

We have `[filter *]` in our Markdown which get's rewritten to jinja2 calls like `{% call filter * %}` which then get evaluated to SSR statements like `[% if format.includes(... %]`.

Instead of relying on the template engine to build the SSR statements [our Markdown extension](https://github.com/ampproject/amp.dev/blob/future/pages/extensions/amp_dev/markdown_extras/block_filter.py) should directly emit them.