Remove unnecessary use of MarkdownElementBuilder::flush_text (#30376)

The next statement calls `modify_current_div` which immediately does
`flush_text`.

Release Notes:

- N/A

crates/markdown/src/markdown.rs

@@ -1048,7 +1048,6 @@ impl Element for MarkdownElement {
                             copy_button: true, ..
                         } = &self.code_block_renderer
                         {
-                            builder.flush_text();
                             builder.modify_current_div(|el| {
                                 let content_range = parser::extract_code_block_content_range(
                                     parsed_markdown.source()[range.clone()].trim(),