[JENKINS-21016] Remove credential section as helper is a property within the credential section.

[JENKINS-21016] correct remove-section.  The section is called credential rather than credential.helper e.g.:

C:\Users\test>git config --local --list
credential.helper=store --file="c:\creden.txt"

C:\Users\test>git config --local --remove-section credential.helper
fatal: No such section!

C:\Users\test>git config --local --remove-section credential

C:\Users\test>git config --local --list