Fix regexp not removing multi-line comments

[thiemowmde]

trafficstars

The previous regular expression had two issues:

1. It was only able to remove 1-line comments.
2. It was not set to be ungreedy, and could potentially remove content between two comments. E.g. when something like <!-- comment --> content <!-- comment --> does not contain a newline character, the content would actually be removed.

We run into this issue at https://gerrit.wikimedia.org/r/489323. As a temporary workaround we made all our comments 1-line comments, and made sure each comment is on a separate line.

[jsf-clabot]

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