fix(snix/eval/value): drop into_bstring

We have a `impl From<NixString> for BString` already.

Change-Id: I278b62ac06ffaf47b2b0ecf394f0bc67213b5c40
Reviewed-on: https://cl.snix.dev/c/snix/+/30226
Autosubmit: Florian Klink <flokli@flokli.de>
Tested-by: besadii
Reviewed-by: Ryan Lahfa <masterancpp@gmail.com>

snix/eval/src/value/string/mod.rs

@@ -638,10 +638,6 @@ impl NixString {
         std::str::from_utf8(self.as_bytes())
     }
 
-    pub fn into_bstring(self) -> BString {
-        self.as_bstr().to_owned()
-    }
-
     /// Return a displayable representation of the string as an
     /// identifier.
     ///

snix/eval/src/vm/mod.rs

@@ -801,12 +801,7 @@ where
                         .unwrap_or(0);
 
                     self.enqueue_generator("resolve_with", op_span, |co| {
-                        resolve_with(
-                            co,
-                            ident.as_bstr().to_owned(),
-                            with_stack_len,
-                            closed_with_stack_len,
-                        )
+                        resolve_with(co, ident.into(), with_stack_len, closed_with_stack_len)
                     });
 
                     return Ok(false);