Remove pom.xml from gRPC KeepAlive example (#2206)

Motivation:
The gRPC KeepAlive example was copy/pasted from helloworld
which brought a pom.xml that isn't necessary and not updated
for this project.

servicetalk-examples/grpc/keepalive/README.adoc

@@ -1,9 +1,9 @@
 == ServiceTalk gRPC KeepAlive Example
 
 Demonstrates how to use HTTP/2 keep alive for gRPC
-link:{source-root}/servicetalk-examples/grpc/keepalive/src/main/java/io/servicetalk/examples/grpc/keepalive/KeepAliveServer.java[server]
+link:src/main/java/io/servicetalk/examples/grpc/keepalive/KeepAliveServer.java[server]
 and
-link:{source-root}/servicetalk-examples/grpc/keepalive/src/main/java/io/servicetalk/examples/grpc/keepalive/KeepAliveClient.java[client].
+link:src/main/java/io/servicetalk/examples/grpc/keepalive/KeepAliveClient.java[client].
 Keep alive uses transport control frames to ensure the peer is still able to read and write to open connections. If the
 peer is not able to respond to the control frame within the configured amount of time, the connection is closed. This
 is useful if your environment doesn't provide other forms of connection keep alive (e.g.