build: Clean the build directory before building the wheel

- When rebuilding the wheel, it had stale files for the _js files, which was confusing and annoying
- Clean the build directory so it doesn't have stale files
- Remove the `package_data` flag - it's already captured by default with the `include_package_data` option: https://setuptools.pypa.io/en/latest/userguide/datafiles.html#include-package-data
- Still need to run `npm run build` between builds

plugins/ui/setup.py

@@ -1,10 +1,17 @@
 from setuptools import setup
 import os
+import shutil
 from deephaven.plugin.packaging import package_js
 
 js_dir = "src/js/"
 dest_dir = os.path.join("src/deephaven/ui/_js")
 
+# remove the build directory to ensure that the package is built from the latest js files
+try:
+    shutil.rmtree("build")
+except FileNotFoundError:
+    pass
+
 package_js(js_dir, dest_dir)
 
-setup(package_data={"deephaven.ui._js": ["**"]})
+setup()