fix: Remove orphaned span_text helper function from markdown test module

The span_text function in src/view/markdown.rs's test module was no longer
called after the dim_false_plain_text_has_no_color test was fixed. With the
crate's dead_code = "deny" lint enabled, this unused function caused a compile
error. Removed it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/view/markdown.rs

@@ -437,10 +437,6 @@ fn wrap_spans_to_lines(spans: &[Span<'static>], target_width: usize) -> Vec<Vec<
 mod tests {
     use super::*;
 
-    fn span_text(spans: &[Span<'static>]) -> String {
-        spans.iter().map(|s| s.content.as_ref()).collect()
-    }
-
     #[test]
     fn dim_false_plain_text_has_no_color() {
         let spans = render_markdown("hello world", 0, false);