removed unused variable in Doubly linked list and amended insert method

[cynx]

append method of doubly linked list had unused current variable - removed same.
in insert method of doubly linked list, for position === 0, we check if its !this.getHead(). however, this.getHead() is already available in current variable, hence instead we can just check !current

[loiane]

In the first file, the 'current' variable is missing.
In the second file, the name of the class was renamed, which will make the test file fail.