Remove unused arg 'defer' in 'job_start' #1000

* With the changes in commit
  "events: Refactor how event deferral is handled"
  (2e4ea29d2c) the function argument
  'defer' of 'job_start' and member variable 'defer' of 'struct job'
  can be removed.
* Update/Fix the documentation for function 'job_start'.

src/nvim/os/shell.c

@@ -280,7 +280,6 @@ int os_system(const char *cmd,
                        system_data_cb,
                        system_data_cb,
                        NULL,
-                       false,
                        0,
                        &i);