Delete empty library crate from cxxbridge-cmd package

This used to be needed in order for cxxbridge-cmd to be vendorable by
cargo vendor, but is superseded by the new "artifact dependencies"
functionality of Cargo.

    [dependencies]
    cxx = "1"
    cxxbridge-cmd = { version = "1", artifact = "bin" }
diff --git a/gen/cmd/src/lib.rs b/gen/cmd/src/lib.rs
deleted file mode 100644
index 8b1a393..0000000
--- a/gen/cmd/src/lib.rs
+++ /dev/null
@@ -1 +0,0 @@
-// empty