CLN: remove dead code in pandas.computation
closes #5041

Coverage:

```
Name                                 Stmts   Miss  Cover   Missing
------------------------------------------------------------------
pandas.computation                       0      0   100%
pandas.computation.align                96      0   100%
pandas.computation.api                   2      0   100%
pandas.computation.common               14      0   100%
pandas.computation.engines              63      6    90%   94, 122-126
pandas.computation.eval                 62      4    94%   39-40, 46, 90
pandas.computation.expr                330      4    99%   512-514, 538-540
pandas.computation.expressions         101      9    91%   106-107, 140-145, 197
pandas.computation.ops                 231      0   100%
pandas.computation.pytables            350      5    99%   37, 122, 165, 202, 233
pandas.computation.scope               103      8    92%   34-36, 72-74, 135-137
pandas.computation.tests                 0      0   100%
pandas.computation.tests.test_eval    1030      4    99%   50-51, 56, 166
------------------------------------------------------------------
TOTAL                                 2382     40    98%
----------------------------------------------------------------------
Ran 1099 tests in 88.447s

OK (SKIP=26)
```
 Most uncovered there are Py3 things
 @jreback You ok with this?
 Obvs need 2 squash
 Alright gonna squash it.
 looks fine
