Remove redundant assignment in win_set_environment_variable. NFC (#1744)

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 b6e86f0..8858ab9 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