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/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> {