Remove to_html stub (#1132)

The recent change that added inline data explorers for R caused a
regression in printing data frames at the R console.

Ark contains a stub for printing data frames that just says `Hello,
world!` Before the PR, it was dead code because the old code looked for
`.Last.value` in `R_GlobalEnv` (where it doesn't exist), while R stores
it in the base environment. The new code correctly uses`
harp::environment::last_value()` which finds it in the base env, so now
the stub actually gets called for data frames.

The fix here is to remove the stub entirely since it doesn't do anything
useful. We can add it back if we decided to implement HTML/tabular
output for data frames properly.

Will address posit-dev/positron#12799.

Author: jmcphers

crates/ark/src/console/console_repl.rs

@@ -1158,18 +1158,10 @@ impl Console {
             return data;
         }
 
-        // If this is a data frame, add HTML representation and open inline explorer
+        // If this is a data frame, optionally open an inline data explorer
         // (only in Positron notebook mode)
         if r_is_data_frame(value.sexp) {
             let value = value.sexp;
-            match to_html(value) {
-                Ok(html) => {
-                    data.insert("text/html".to_string(), json!(html));
-                },
-                Err(err) => {
-                    log::error!("{err:?}");
-                },
-            };
 
             // The inline data explorer is a Positron-specific feature that
             // requires comm support. Other Jupyter frontends don't understand
@@ -2426,15 +2418,6 @@ impl Console {
     }
 }
 
-/// Converts a data frame to HTML
-fn to_html(frame: SEXP) -> Result<String> {
-    let result = RFunction::from(".ps.format.toHtml")
-        .add(frame)
-        .call()?
-        .to::<String>()?;
-    Ok(result)
-}
-
 // Inputs generated by `ReadConsole` for the LSP
 pub(crate) fn console_inputs() -> anyhow::Result<ConsoleInputs> {
     // TODO: Should send the debug environment if debugging: