Remove check for collection type from the strip_title_exerpt? function

[staticintlucas]

Remove the check for collection type when determining whether to strip the title from the excerpt. Fixes #81

I'm not super familiar with Jekyll so not sure if this will have any unintended side effects. It seemed to work fine in my testing though.

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[benbalter]

It looks like this was implemented in https://github.com/benbalter/jekyll-titles-from-headings/pull/28. @qwtel do you recall why posts are special cased and what change in behavior, if any, removing it might have?

[staticintlucas]

@qwtel @benbalter Any movement on this?

From what I can tell the only difference between posts and any other collection is that by default posts has output: true to generate individual pages. But this is already taken into account by iterating over site.pages which will not include any collections with output: false.

FWIW my original issue would also have been fixed by PR #64, so if you prefer you could merge that instead.