BUG: linalg: Remove a couple shadowed (and buggy) tests from test_decomp.py.
MAINT: linalg: Remove some unused variables from test_decomp.py.
MAINT: sparse: Clean up in bench_sparse.py:

* Remove unused imports.
* Don't use 'import *' form of import.
MAINT: sparse: Remove unused imports and unused variables.
MAINT: sparse: Remove some more unused imports and unused variables.
Merge pull request #4250 from WarrenWeckesser/pyflakes-cleanup

MAINT: Some pyflakes-driven cleanup in linalg and sparse