remove mimalloc allocator (#15317)

# Description

This PR removes the mimalloc allocator due to run-away memory leaks
recently found.

closes #15311

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crates/nu-cmd-lang/Cargo.toml

@@ -42,7 +42,6 @@ plugin = [
   "os",
 ]
 
-mimalloc = []
 trash-support = []
 sqlite = []
 static-link-openssl = []

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

@@ -161,11 +161,7 @@ fn push_version_numbers(record: &mut Record, head: Span) {
 }
 
 fn global_allocator() -> &'static str {
-    if cfg!(feature = "mimalloc") {
-        "mimalloc"
-    } else {
-        "standard"
-    }
+    "standard"
 }
 
 fn features_enabled() -> Vec<String> {

crates/nu_plugin_polars/Cargo.toml

@@ -27,7 +27,6 @@ chrono = { workspace = true, features = ["std", "unstable-locales"], default-fea
 chrono-tz = "0.10"
 fancy-regex = { workspace = true }
 indexmap = { version = "2.7" }
-mimalloc = { version = "0.1.42" }
 num = {version = "0.4"}
 serde = { version = "1.0", features = ["derive"] }
 sqlparser = { version = "0.53"}

crates/nu_plugin_polars/src/main.rs

@@ -1,9 +1,6 @@
 use nu_plugin::{serve_plugin, MsgPackSerializer};
 use nu_plugin_polars::PolarsPlugin;
 
-#[global_allocator]
-static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc;
-
 fn main() {
     env_logger::init();