Remove missed async keywords from the revert

This should've been in 7d21b40401.
This completes the revert of async sessions.

telethon/sessions/string.py

@@ -11,15 +11,6 @@ _STRUCT_PREFORMAT = '>B{}sH256s'
 CURRENT_VERSION = '1'
 
 
-class _AsyncString(str):
-    """
-    Ugly hack to enable awaiting strings.
-    """
-    def __await__(self):
-        yield
-        return self
-
-
 class StringSession(MemorySession):
     """
     This session file can be easily saved and loaded as a string. According
@@ -59,17 +50,14 @@ class StringSession(MemorySession):
         return base64.urlsafe_b64decode(x)
 
     def save(self: Session):
-        # save should be async but that would break code which relies on sync StringSession.
-        # Return a type which behaves like a string for all intents and purposes, but can be awaited.
-        # The await is necessary for the library to save all sessions in the same way.
         if not self.auth_key:
-            return _AsyncString('')
+            return ''
 
         ip = ipaddress.ip_address(self.server_address).packed
-        return _AsyncString(CURRENT_VERSION + StringSession.encode(struct.pack(
+        return CURRENT_VERSION + StringSession.encode(struct.pack(
             _STRUCT_PREFORMAT.format(len(ip)),
             self.dc_id,
             ip,
             self.port,
             self.auth_key.key
-        )))
+        ))