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-cmd-lang/Cargo.toml

@@ -28,6 +28,5 @@ mimalloc = []
 which-support = []
 trash-support = []
 sqlite = []
-dataframe = []
 static-link-openssl = []
 system-clipboard = []

crates/nu-cmd-lang/README.md

@@ -8,7 +8,6 @@ top of including:
 
 * nu-command
 * nu-cli
-* nu-cmd-dataframe
 * nu-cmd-extra
 
 As time goes on and the nu language develops further in parallel with nushell we will be adding other command crates to the system.

crates/nu-cmd-lang/src/core_commands/version.rs

@@ -175,11 +175,6 @@ fn features_enabled() -> Vec<String> {
         names.push("sqlite".to_string());
     }
 
-    #[cfg(feature = "dataframe")]
-    {
-        names.push("dataframe".to_string());
-    }
-
     #[cfg(feature = "static-link-openssl")]
     {
         names.push("static-link-openssl".to_string());