src: remove unused argc var in node_stat_watcher

Currently the following compiler warning is reported:
../src/node_stat_watcher.cc:113:13: warning:
unused variable 'argc' [-Wunused-variable]
  const int argc = args.Length();
            ^
1 warning generated.

This commit removes the unused argc variable.

PR-URL: https://github.com/nodejs/node/pull/21337


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>