process_stop: uv: do not close stdin first/explicitly #10584

- process_stop: do not close stdin explicitly. The "close stdin" step was from
  aa9cb48, before we fixed/reworked the SIGTERM timing logic. So it's probably
  outdated / no longer needed.
- win: jobstop: exit_code 15
   GetExitCodeProcess appears to return the used signal.
   https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-getexitcodeprocess

ref #10573

src/nvim/event/process.c

@@ -223,10 +223,6 @@ void process_stop(Process *proc) FUNC_ATTR_NONNULL_ALL
 
   switch (proc->type) {
     case kProcessTypeUv:
-      // Close the process's stdin. If the process doesn't close its own
-      // stdout/stderr, they will be closed when it exits(possibly due to being
-      // terminated after a timeout)
-      stream_may_close(&proc->in);
       os_proc_tree_kill(proc->pid, SIGTERM);
       break;
     case kProcessTypePty: