debugger: Remove PHP debug adapter (#34020)

This commit removes the PHP debug adapter in favor of a new version
(0.3.0) of PHP extension.
The name of a debug adapter has been changed from "PHP" to "Xdebug",
which makes this a breaking change in user-configured scenarios

Release Notes:

- debugger: PHP debug adapter is no longer shipped in core Zed editor;
it is now available in PHP extension (starting with version 0.3.0). The
adapter has been renamed from `PHP` to `Xdebug`, which might break your
user-defined debug scenarios.

assets/settings/initial_debug_tasks.json

@@ -3,13 +3,6 @@
 // For more documentation on how to configure debug tasks,
 // see: https://zed.dev/docs/debugger
 [
-  {
-    "label": "Debug active PHP file",
-    "adapter": "PHP",
-    "program": "$ZED_FILE",
-    "request": "launch",
-    "cwd": "$ZED_WORKTREE_ROOT"
-  },
   {
     "label": "Debug active Python file",
     "adapter": "Debugpy",