Pyflakes fixes for core files

These were mostly just unused imports. There was an unused branch

if isinstance(p, decimal.Decimal):
    rv =  _decimal_to_Rational_prec(p)[0]

in numbers.py.  I also found an undefined name _clear() in sympify.py that I
did not know how to handle, so I opened
https://code.google.com/p/sympy/issues/detail?id=3818 for it.