Remove emsdk_set_ene.bat from .gitignore (#550)

This file should only ever exist momentarily.  Its written during
`emsdk construct_env` on windows, but emsdk.bat immediately uses
at deletes it before returning.

Having this file remain around on disk would be a bug so we shouldn't
ignore its presense.

.gitignore

@@ -9,9 +9,6 @@ __pycache__
 /.emscripten_sanity
 /.emscripten_sanity_wasm
 
-# Auto-generated by emsdk.py under windows by `construct_env` or `activate`
-/emsdk_set_env.bat
-
 # Tags files that get generated at runtime
 /emscripten-releases-tot.txt