build: remove unused swift plugin file and update error message

NEWS

@@ -16,6 +16,7 @@ What's New in libchewing (unreleased)
   - New boolean config option `chewing.sort_candidates_by_frequency` can be
     set to enable sorting candidates by frequency.
   - Rust type SyllableSlice is removed from public interfaces.
+  - Provide libchewing as a Swift package (C based API only).
 
 * Bug Fixes
   - Auto shift cursor after candidate selection now moves the cursor to the

swift/tools/CargoBuildPlugin/Plugin.swift

@@ -39,7 +39,7 @@ struct CargoBuildPlugin: BuildToolPlugin {
         init(_ s: String) { description = s }
       }
       throw UserError(
-        "`cargo` not found on the system. Please install Rust (https://rustup.rs/) to enable automatic builds, or disable automatic Cargo build by setting `LIBCHEWING_AUTO_BUILD_CARGO=0` and run `swift/scripts/build-cargo.sh` manually to produce the library before running `swift build`."
+        "`cargo` not found on the system. Please install Rust (https://rustup.rs/) to enable automatic builds, or disable automatic Cargo build by setting `LIBCHEWING_AUTO_BUILD_CARGO=0` and manually compile the Rust library before running `swift build`."
       )
     }