Fix test_pytest functions with the same name

raises() with a string is no longer supported, so delete those tests.
Remove unused variables from autowrap.py

The variable prototype_c has never been used since the commit that introduced
it, 2dbb3fbea27c82a669975edafd01156c245fa984.
Pyflakes on utilities

Mostly just unused names. There are still a few false positives from globals
and some logic that pyflakes can't handle.
Pyflakes on utilities tests

I left the unused imports that represented things that should be tested.

One change was a variable name typo (in code_quality). The rest were unused
variables or imports.
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.
Merge pull request #2096 from asmeurer/pyflakes

Pyflakes fixes for various parts of SymPy.