[macros] Remove pkg/_macros and pkg/macros

Change-Id: I14bde57b4de1a9dafe9aa291baf0fffca89c3b19
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420680
Reviewed-by: Devon Carew <devoncarew@google.com>
Reviewed-by: Jake Macdonald <jakemac@google.com>

sdk_packages.yaml

@@ -2,6 +2,3 @@
 version: 1
 sdk: dart
 # Note that paths are relative to the root of the built SDK.
-packages:
-  - name: _macros
-    path: pkg/_macros