Remove dataframes crate and feature (#12889)

# Description
Removes the old `nu-cmd-dataframe` crate in favor of the polars plugin.
As such, this PR also removes the `dataframe` feature, related CI, and
full releases of nushell.

crates/nu-command/tests/commands/open.rs

@@ -238,22 +238,6 @@ fn parses_xml() {
     assert_eq!(actual.out, "https://www.jntrnr.com/off-to-new-adventures/")
 }
 
-#[cfg(feature = "dataframe")]
-#[test]
-fn parses_arrow_ipc() {
-    let actual = nu!(
-        cwd: "tests/fixtures/formats", pipeline(
-        "
-            dfr open caco3_plastics.arrow
-            | dfr into-nu
-            | first
-            | get origin
-        "
-    ));
-
-    assert_eq!(actual.out, "SPAIN")
-}
-
 #[test]
 fn errors_if_file_not_found() {
     let actual = nu!(

crates/nu-command/tests/main.rs

@@ -151,9 +151,8 @@ fn commands_declare_input_output_types() {
         let sig_name = cmd.signature().name;
         let category = cmd.signature().category;
 
-        if matches!(category, Category::Removed | Category::Custom(_)) {
+        if matches!(category, Category::Removed) {
             // Deprecated/Removed commands don't have to conform
-            // TODO: also upgrade the `--features dataframe` commands
             continue;
         }