[postprocessor:exec] do not auto-add '{}' to command (#1185)

This was initially done to mimic youtube-dl's behavior and
implementation of --exec, and it seemed reasonable at the time.

gallery_dl/postprocessor/exec.py

@@ -30,8 +30,6 @@ class ExecPP(PostProcessor):
 
         args = options["command"]
         if isinstance(args, str):
-            if "{}" not in args:
-                args += " {}"
             self.args = args
             execute = self.exec_string
         else: