minor: removes dead code in diff.groovy

[rnveach]

Identified at #747 (comment) and moved off for quicker merge

[rnveach]

@nrmancuso ping

./.ci/travis.sh: line 119: mdl: command not found

and other failures.

[nrmancuso]

@nrmancuso ping

./.ci/travis.sh: line 119: mdl: command not found

and other failures.

All is addressed in #746

[rnveach]

https://app.circleci.com/pipelines/github/checkstyle/contribution/21/workflows/88fb3974-c3d0-45f9-a103-78e63ab804d8/jobs/125

Switched to a new branch 'patch-branch'
Caught: groovy.lang.MissingPropertyException: No such property: checkstyleToolBaseConfig for class: Config
groovy.lang.MissingPropertyException: No such property: checkstyleToolBaseConfig for class: Config

This is a real failure.

[rnveach]

Closing. The methods are actually used just not in their full form.

getDiffToolConfig:
https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/diff.groovy#L40
getCheckstyleToolPatchConfig:
https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/diff.groovy#L34
getCheckstyleToolBaseConfig:
https://github.com/checkstyle/contribution/blob/master/checkstyle-tester/diff.groovy#L31

I will need to look over these compared to my new changes