tools: enable no-cond-assign-ESLint rule

no-cond-assign is one of the few ESLint recommended rules that we have
turned off. This change enables the rule.

PR-URL: nodejs#41614
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

.eslintrc.js

@@ -295,7 +295,6 @@ module.exports = {
     'valid-typeof': ['error', { requireStringLiterals: true }],
 
     // ESLint recommended rules that we disable
-    'no-cond-assign': 'off',
     'no-empty': 'off',
     'no-inner-declarations': 'off',
     'no-prototype-builtins': 'off',