[generator] remove unused function

I'm planning on refactoring this code, let's get a trivial change out of the way first

Author: shish

generator/src/PhpStanFunctions/PhpStanType.php

@@ -214,11 +214,6 @@ public function isNullable(): bool
         return $this->nullable;
     }
 
-    public function isFalsable(): bool
-    {
-        return $this->falsable;
-    }
-
     /**
      * @return array<string>
      */

generator/tests/PhpStanFunctions/PhpStanTypeTest.php

@@ -88,7 +88,6 @@ public function testParenthesisOutsideOfCallable(): void
     public function testFalsable(): void
     {
         $param = new PhpStanType('string|false');
-        $this->assertEquals(true, $param->isFalsable());
         $this->assertEquals('string|false', $param->getDocBlockType());
         $this->assertEquals('string', $param->getSignatureType());
     }