Remove some usages of StringUtils

[basil]

Start to remove some low-hanging fruit.

Testing done

mvn clean verify

[BobDu]

Some additional thoughts.

run in jenkins-usage-in-plugins search the use of the json-lib class.

cat addclass.txt 

net.sf.json.*
net.sf.ezmorph.*

mvn clean compile exec:java -Dexec.args="--includeCore -i -C addclass.txt"

Result report:
usage-by-api.json

The XML Serializer related parts seem to be unused.
Can we safe simply removing the XML to reduce the amount of code that requires maintenance? This would also further decrease the API surface area.

[basil]

Probably? Need to run usage-in-plugins transitively with -p, and against CloudBees plugins. In any case I don't see how that is related to this PR. It can be done separately if desired.

[BobDu]

Simply because, if we have the option to remove XMLSerializer, then the modifications to the
XMLSerializer.java file in this PR would be unnecessary.

Of course, regardless, for this PR, LGTM.