fix(tvix/eval): drop superfluous string context check

cl/11712 simultaneously introduced this check and made it unnecessary,
since NixString::context should never return `Some` for empty contexts
now.

Change-Id: I41a655ff33910e8326cbb7d7526eb91bd19e9585
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11713
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/value/string.rs

@@ -654,10 +654,7 @@ impl NixString {
     {
         Self::new(
             Self::from(new_contents).as_ref(),
-            other
+            other.context().map(|c| Box::new(c.clone())),
-                .context()
-                .filter(|ctx| !ctx.is_empty())
-                .map(|c| Box::new(c.clone())),
         )
     }