Remove plexus-java overrides.

These override dates from cl/177360283 and cl/179690614, when they were perhaps necessary for Java 9 support?

Our other projects doing fine with the default `plexus-java` version that `maven-compiler-plugin` pulls in nowadays, so I doubt that we need this configuration.

So let's remove it and give Dependabot one less reason to send us [updates](#482).

RELNOTES=n/a
PiperOrigin-RevId: 744749356

Author: cpovirk

pom.xml

@@ -112,13 +112,6 @@
           <showWarnings>true</showWarnings>
           <showDeprecation>true</showDeprecation>
         </configuration>
-        <dependencies>
-          <dependency>
-            <groupId>org.codehaus.plexus</groupId>
-            <artifactId>plexus-java</artifactId>
-            <version>1.5.0</version>
-          </dependency>
-        </dependencies>
       </plugin>
       <plugin>
         <artifactId>maven-release-plugin</artifactId>