Resolve "Reconfiguration without relative urls does not remove 'relative_url.rb'"

Since version 9.3.0, we are not using relative_url.rb file for configuring relative URLs, but an env variable. This was done with !1381 (merged).

However, this caused the issue of stale relative_url.rb files in a specific scenario. When someone used relative URLs before version 9.3.0, upgraded to 9.3.0 and then removed the usage of relative url. In that case, the relative_url.rb will present, causing this issue.

This MR explicitly removes the file, that may be leftover from previous versions. Relatives URLs will still work, as it is handled by the env variable.

Closes #2675 (closed)