DERBY-6638 Remove unnecessary use of reflection in SignatureChecker

Remove the reflection + some missing javadocs and IDE warning fixes.

Removes unused argument from method "checkSignature", and changes all
StringBuilder usage to avoid concatenation.

I tested the tool manually for a simple case, but see DERBY-6639.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1605738 13f79535-47bb-0310-9956-ffa450edef68