Remove redundant jackson-core override from example app

The direct jackson-core:2.16.0 dependency was creating a version mismatch
with jackson-databind:2.13.5 (managed by the Spring Boot 2.6.15 BOM via
Retrofit's converter-jackson). This pin has been present since the initial
commit and was never intentional — it just drifted as Spring Boot was
updated independently.

Removing it lets Spring Boot's BOM manage all jackson artifacts
consistently. A follow-up Spring Boot 3.x upgrade will address the
remaining Dependabot alerts including the jackson-core async parser DoS
vulnerability.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AaronAtDuo

duo-example/pom.xml

@@ -74,12 +74,6 @@
             <version>4.6.1</version>
             <scope>provided</scope>
         </dependency>
-
-        <dependency>
-            <groupId>com.fasterxml.jackson.core</groupId>
-            <artifactId>jackson-core</artifactId>
-            <version>2.16.0</version>
-        </dependency>
     </dependencies>
 
     <build>