Remove gRPC 3rd party starter reference

Closes gh-50506

Author: snicoll

starter/README.adoc

@@ -119,9 +119,6 @@ do as they were designed before this was clarified.
 | https://javaee.github.io/grizzly/[Grizzly]
 | https://github.com/dabla/grizzly-spring-boot-starter
 
-| https://www.grpc.io/[gRPC]
-| https://github.com/LogNet/grpc-spring-boot-starter & https://github.com/yidongnan/grpc-spring-boot-starter & https://github.com/DanielLiu1123/grpc-starter
-
 | https://ha-jdbc.github.io/[HA JDBC]
 | https://github.com/lievendoclo/hajdbc-spring-boot