Remove unused dependency from pubspec.yaml (#2592)

diff --git a/pubspec.yaml b/pubspec.yaml
index fcf8913..42bbd64 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -34,4 +34,3 @@
   test: ^1.3.0
   test_descriptor: ^1.0.0
   test_process: ^1.0.0
-  protoc_plugin: <19.0.0