refactor(tvix/eval): remove Error::InvalidKeyType

We're confident that we're handling all branches that can reasonably
occur from valid AST, any other cases should be considered a critical
evaluator bug and panic rather than surfacing something that looks
like user error.

Change-Id: If96966eb32b8ff12fcaeb9ea3b0c8fc51b6abd11
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6205
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/errors.rs

@@ -6,10 +6,6 @@ pub enum Error {
         key: String,
     },
 
-    InvalidKeyType {
-        given: &'static str,
-    },
-
     AttributeNotFound {
         name: String,
     },