hive-sh4re: drop the nix explanation from the panic message too

Same layering rule as the previous commit, one construct over. The
remediation text explained how nix wires the var and pointed at
`nix/assets.nix` -- a path that does not exist either, so it carried
both defects this branch is about.

What a developer hitting this panic needs is the layout to point at,
which is a fact about this crate.

hive-sh4re/src/assets.rs

@@ -34,12 +34,10 @@ fn dir() -> PathBuf {
     match std::env::var("HIVE_ASSETS_DIR") {
         Ok(v) if !v.is_empty() => PathBuf::from(v),
         _ => panic!(
-            "HIVE_ASSETS_DIR is not set. Inside the nix-built systemd \
-             units this is wired automatically from \
-             `pkgs.hyperhive-assets`; for `cargo run` outside nix, \
-             set it to a directory laid out like \
-             `nix/assets.nix`'s output (branding/ + prompts/ \
-             subdirs). See hive-sh4re/src/assets.rs for the contract.",
+            "HIVE_ASSETS_DIR is not set. A deployment sets it; when \
+             running a binary outside one, point it at a directory \
+             with `branding/` and `prompts/` subdirs. See \
+             hive-sh4re/src/assets.rs for the contract.",
         ),
     }
 }