Remove unused internal From<&TempDir> implementation (#739)

Since `pack::BuildpackReference` is an internal-only type (not to be
confused with the public `build_config::BuildpackReference`), and the
`From<&TempDir>` implementation is unused since #666.

GUS-W-14502304.

libcnb-test/src/pack.rs

@@ -1,7 +1,6 @@
 use std::collections::BTreeMap;
 use std::path::PathBuf;
 use std::process::Command;
-use tempfile::TempDir;
 
 /// Represents a `pack build` command.
 #[derive(Clone, Debug)]
@@ -27,12 +26,6 @@ impl From<PathBuf> for BuildpackReference {
     }
 }
 
-impl From<&TempDir> for BuildpackReference {
-    fn from(path: &TempDir) -> Self {
-        Self::Path(path.path().into())
-    }
-}
-
 impl From<String> for BuildpackReference {
     fn from(id: String) -> Self {
         Self::Id(id)