Remove uneeded str. NFC (emscripten-core#1170)

These function already returns a string.

emsdk.py

@@ -2709,7 +2709,7 @@ def expand_sdk_name(name, activating):
       if installed:
         debug_print('activating currently installed SDK; not updating tot version')
         return 'sdk-releases-%s-64bit' % installed
-    return str(find_tot_sdk())
+    return find_tot_sdk()
 
   if '-upstream' in name:
     name = name.replace('-upstream', '')