Clean up stray vendor directory at package root during build

On Windows (winbuilder), the tar extraction of vendored Rust crates
creates a vendor/ directory at the package root, causing an R CMD check
NOTE. Add rm -Rf ../vendor to both Makevars.in and Makevars.win to
ensure it is cleaned up after the build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bnaras

src/Makevars.in

@@ -53,6 +53,7 @@ $(STATLIB):
 	  fi
 	rm -Rf $(CARGOTMP)
 	rm -Rf $(VENDOR_DIR)
+	rm -Rf ../vendor
 	rm -Rf $(LIBDIR)/build
 
 clean_intermediate: $(SHLIB)

src/Makevars.win

@@ -34,6 +34,7 @@ $(STATLIB):
 	  cargo build -j 2 --target $(TARGET) --lib --offline --release --manifest-path ./rust/Cargo.toml --target-dir $(TARGET_DIR)
 	rm -Rf $(CARGOTMP)
 	rm -Rf $(VENDOR_DIR)
+	rm -Rf ../vendor
 	rm -Rf $(LIBDIR)/build
 
 clean_intermediate: $(SHLIB)