See if `truncate_source` can be removed

[mre]

In #1832, we moved to Cow for InputSource::String.

We should consider removing this method or alternatively updating the TODO:

lychee/lychee-lib/src/utils/request.rs

Lines 115 to 120 in d6b16bc

	/// Truncate the source in case it gets too long
	///
	/// This is only needed for string inputs.
	/// For other inputs, the source is simply a "label" (an enum variant).
	// TODO: This would not be necessary if we used `Cow` for the source.
	fn truncate_source(source: &InputSource) -> InputSource {

Originally posted by @katrinafyi in #1832 (comment)