Remove unused showDeprecation parameter

[basil]

Problem

Building a plugin with Maven 4.0.0-alpha-3 results in (among other problems)

[INFO] ------------------------------------------< org.jenkins-ci.plugins:text-finder >------------------------------------------
[INFO] Building Text Finder 1.23-SNAPSHOT
[INFO]   from pom.xml
[INFO] ---------------------------------------------------------[ hpi ]----------------------------------------------------------
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:validate (default-validate)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:validate-hpi (default-validate-hpi)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:insert-test (default-insert-test)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:test-hpl (default-test-hpl)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:resolve-test-dependencies (default-resolve-test-dependencies)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:test-runtime (default-test-runtime)'
[WARNING] Parameter 'showDeprecation' is unknown for plugin 'maven-hpi-plugin:3.38:hpi (default-hpi)'

Evaluation

This code was originally added in jenkinsci/jenkins@5dfb1ad. At the time, compilation was handled by maven-hpi-plugin rather than by maven-compiler-plugin. Compilation was switched back to the standard maven-compiler-plugin in jenkinsci/maven-hpi-plugin@7c3969e, and the mojo with the showDeprecation option was deleted in jenkinsci/maven-hpi-plugin@334506f, but the configuration parameter remained. Specifying this parameter seems to have been ignored in Maven 3.x but is now showing a warning in Maven 4.x.

Solution

Remove the unused parameter.

Testing done

Verified that the warning was no longer shown after this parameter was removed and that compilation still succeeded.