fix(tvix/eval): don't impl From<NixString> for String

This caught me by accident in an earlier revision of cl/11500 -
I had a `NixString`, wanted to return it as a `String`, so I was naively
calling `s.into()`.

That unfortunately gave me the `Display` implementation of `NixString`,
which quotes strings, causing an annoying error further up the stack.

NixStrings are bytes, we can keep the impl From<NixString> for BString,
but having a `.into()` suddenly do quoting is more than unexpected.

Change-Id: I5434ba94bfe6c493d0a57e68225ecc22daa4b948
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11505
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/value/string.rs

@@ -530,11 +530,7 @@ impl<'a> From<&'a NixString> for &'a BStr {
     }
 }
 
-impl From<NixString> for String {
-    fn from(s: NixString) -> Self {
-        s.to_string()
-    }
-}
+// No impl From<NixString> for String, that one quotes.
 
 impl From<NixString> for BString {
     fn from(s: NixString) -> Self {