Chore: remove dead code from prefer-const (#8683)

This code was added to work around a bug in acorn (acornjs/acorn#487), which has since been fixed.

lib/rules/prefer-const.js

@@ -85,17 +85,6 @@ function getIdentifierIfShouldBeConst(variable, ignoreReadBeforeAssign) {
         return null;
     }
 
-    /*
-     * Due to a bug in acorn, code such as `let foo = 1; let foo = 2;` will not throw a syntax error. As a sanity
-     * check, make sure that the variable only has one declaration. After the parsing bug is fixed, this check
-     * will no longer be necessary, because variables declared with `let` or `const` should always have exactly one
-     * declaration.
-     * https://github.com/ternjs/acorn/issues/487
-     */
-    if (variable.defs.length > 1) {
-        return null;
-    }
-
     // Finds the unique WriteReference.
     let writer = null;
     let isReadBeforeInit = false;