Remove unused ManuScript code that does not work any more

[th-we]

This unused code won't work any more because the surrounding code has evolved away from it. Specifically, MEIFlattened is no longer a SparseArray and can not be used in the way these function use it.

[th-we]

@ahankinson Can we merge this? To sum up the changes:

• getPositionInDocument() and lookBack() were removed. They do not work any more because they assume the global MEIFlattened is a SparseArray, but it's not a Dictionary. They are not used anyway in either libmei.plg or sibmei.
• XML parser is "parked" in a separate Python variable, documenting the state of the code.