remove: remove math/base/ops/imul

This commit removes `@stdlib/math/base/ops/imul` in favor of
`@stdlib/number/int32/base/mul`.

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

To migrate, users should update their require/import paths to use
`@stdlib/number/int32/base/mul` which provides the same API and
implementation.

Ref: #2261

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Author: gururaj1512