Adapt GitTool.DescriptorImpl.doCheckHome for Jenkins 1.563

The doCheckHome method was declared to throw an exception, but this
implementation does not throw any exception.  Declaring that exception
breaks compilation with Jenkins 1.563.  Removing the exception throw
also seems to work with older Jenkins versions.
[Fix JENKINS-23017] delete the tmp workdir also

In order to not leave unlimited directories created by the polling cycle
in the tmp directory which results in reaching the system limit for
subdirectory numbers we should also delete the workdir itself and not ony its
contents.