[PHP] remove PersistentChannelTest.testInitHelper (grpc#38042)

Basic Tests PHP MacOS  fails with
```
There was 1 risky test:

1) PersistentChannelTest::testInitHelper
This test did not perform any assertions

/[Volumes/BuildData/tmpfs/altsrc/github/grpc/workspace_php8_macos_dbg_native/src/php/tests/unit_tests/PersistentChannelTests/PersistentChannelTest.php:54](https://cs.corp.google.com/piper///depot/google3/Volumes/BuildData/tmpfs/altsrc/github/grpc/workspace_php8_macos_dbg_native/src/php/tests/unit_tests/PersistentChannelTests/PersistentChannelTest.php?l=54&ws&snapshot=0)

OK, but incomplete, skipped, or risky tests!
Tests: 20, Assertions: 84, Risky: 1.
```

Closes grpc#38042

COPYBARA_INTEGRATE_REVIEW=grpc#38042 from HannahShiSFB:remove-php-risky-test 022307e
PiperOrigin-RevId: 691953016

src/php/tests/unit_tests/PersistentChannelTests/PersistentChannelTest.php

@@ -51,14 +51,6 @@ public function assertConnecting($state) {
       $state == GRPC\CHANNEL_TRANSIENT_FAILURE);
   }
 
-  public function testInitHelper()
-  {
-      // PersistentList is not empty at the beginning of the tests
-      // because phpunit will cache the channels created by other test
-      // files.
-  }
-
-
   public function testChannelNotPersist()
   {
       $this->channel1 = new Grpc\Channel('localhost:1', ['force_new' => true]);