remove: remove math/base/ops/cmul

This commit removes `@stdlib/math/base/ops/cmul` in favor of
`@stdlib/complex/float64/base/mul`.

BREAKING CHANGE: remove `math/base/ops/cmul`

To migrate, users should update their require/import paths to use
`@stdlib/complex/float64/base/mul` which provides the same API and
implementation.

Ref: #2260

Author: kgryte