Remove slf4j as dependencies (#4233)

From the kafka-schema-registry-client in the avro extension slf4j
will be packaged into the distribution. We don't want this as it
will conflict and throw a slf4j multiple bindings warning.
This will cause slf4j to fall back to no-operation (NOP) binding.

extensions-core/avro-extensions/pom.xml

@@ -79,6 +79,12 @@
       <groupId>io.confluent</groupId>
       <artifactId>kafka-schema-registry-client</artifactId>
       <version>${confluent.version}</version>
+      <exclusions>
+        <exclusion>
+          <groupId>org.slf4j</groupId>
+          <artifactId>slf4j-log4j12</artifactId>
+        </exclusion>
+      </exclusions>
     </dependency>
     <dependency>
       <groupId>org.apache.hadoop</groupId>