Consider excluding UnnecessaryGetter in recommended-jenkinsfile

[felipecrs]

Since we don't create classes for declarative pipelines, this rule does not make sense. Often, all we have is a function defined globally:

boolean isTrue(boolean var) {
  return isTrue
}

pipeline {
  [...]
}

This rule would complain about the isTrue function name.

[felipecrs]

I just created this issue so I can mention it in the PR.