mouse-test: Remove useless unreachable closing markup in GEGL message

This markup isn't needed since it's already added later in the function. Also, it was ignored because of some dark magic with the comma operator (which shouldn't have been there to begin with). That's why it wasn't discovered earlier. See https://en.cppreference.com/w/c/language/operator_other#Comma_operator for some explanation on how the comma operator works.

CC @felipeborges (mouse panel maintainer)