pw_protobuf_compiler: Remove legacy generator targets

This removes the legacy <target_name>_<generator> targets from
proto.gni.

Change-Id: I63d9897387f3673ac443fa13641cad62a012b104
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/22664
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Alexei Frolov <frolv@google.com>
diff --git a/pw_protobuf_compiler/proto.gni b/pw_protobuf_compiler/proto.gni
index e3da252..6878156 100644
--- a/pw_protobuf_compiler/proto.gni
+++ b/pw_protobuf_compiler/proto.gni
@@ -497,12 +497,6 @@
                    "generator '$_gen'. Please add support if you require it.",
                  ]))
     }
-
-    # TODO(frolv): Remove this after migrating existing code.
-    _legacy_target = "${target_name}_${_gen}"
-    group(_legacy_target) {
-      public_deps = [ ":$_lang_target" ]
-    }
   }
 
   # All supported pw_protobuf generators.