MAINT: remove unused simple_capsule_dtor() from npy_3kcompat.h

This removes a lot of build warnings when including this file.
MAINT: remove unused variables from einsum.c.src and loops.c.src

Fixes some build warnings.
MAINT: fix build warnings for umath_linalg

The removal of CFLOAT for eig/eigvals is related to a bug, see c355550dd80.
Can be put back once that issue is fixed.
MAINT: fix build warnings in test_rational.c.src
MAINT: silence some build warnings due to missing NO_IMPORT_ARRAY
Merge pull request #4212 from rgommers/buildwarnings

Fix some build warnings