Revert "repl: always check for NODE_REPL_MODE environment variable"

This reverts commit b831b081.

This presumably unbreaks the ASAN github action build.

Example failure:

    2020-06-25T19:59:15.1448178Z === release test-repl-envvars ===
    2020-06-25T19:59:15.1448872Z Path: parallel/test-repl-envvars
    2020-06-25T19:59:15.1449449Z --- stderr ---
    2020-06-25T19:59:15.1449835Z assert.js:103
    2020-06-25T19:59:15.1450194Z   throw new AssertionError(obj);
    2020-06-25T19:59:15.1450524Z   ^
    2020-06-25T19:59:15.1450817Z
    2020-06-25T19:59:15.1451431Z AssertionError [ERR_ASSERTION]: Expected values to be strictly deep-equal:
    2020-06-25T19:59:15.1452000Z + actual - expected
    2020-06-25T19:59:15.1452298Z
    2020-06-25T19:59:15.1452634Z   {
    2020-06-25T19:59:15.1452978Z     terminal: true,
    2020-06-25T19:59:15.1453321Z +   useColors: false
    2020-06-25T19:59:15.1453861Z -   useColors: true
    2020-06-25T19:59:15.1454225Z   }
    2020-06-25T19:59:15.1454841Z     at /home/runner/work/node/node/test/parallel/test-repl-envvars.js:55:12
    2020-06-25T19:59:15.1455246Z     at internal/repl.js:33:5

PR-URL: https://github.com/nodejs/node/pull/34058


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>