remove: remove stats/base/nanvariance

This commit removes `@stdlib/stats/base/nanvariance` in favor of `@stdlib/stats/strided/nanvariance`.

BREAKING CHANGE: remove `stats/base/nanvariance`

To migrate, users should update their require/import paths to use
`@stdlib/stats/strided/nanvariance`, which provides the same API and implementation.

Ref: #4797

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
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  - task: lint_javascript_tests
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  - task: lint_typescript_declarations
    status: na
  - task: lint_typescript_tests
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  - task: lint_license_headers
    status: passed
---

Author: gururaj1512