STYLE: Miscellaneous import cleanup in tract_math script (#81)

Miscellaneous import cleanup in `tract_math` script:
- Remove unused imports: do not import `os` and `warning`.
- Remove duplicate imports: remove duplicate `sys` import statement.

Take advantage of the commit to remove the commented statement appending
the parent directory to the path.

scripts/tract_math

@@ -5,11 +5,6 @@ from argparse import ArgumentParser, FileType, REMAINDER
 
 import traceback
 
-import warnings
-
-import os, sys
-# sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
-
 from tract_querier.tract_math import operations