Remove redundant assignment in win_set_environment_variable. NFC

The assignment of the return code of the process to the local
value variable here doesn't make sense and also doesn't do anything.

This code can be traced all the way back to 2ed7a605a9 but IIUC
that assignment was meaning less even then.

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index 930792f..da6afe0 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -383,7 +383,7 @@
       else:
         cmd = ['REG', 'DELETE', 'HKCU\\Environment', '/V', key, '/f']
       debug_print(str(cmd))
-      value = subprocess.call(cmd, stdout=subprocess.PIPE)
+      subprocess.call(cmd, stdout=subprocess.PIPE)
     except Exception:
       return False
     return True