remove sparse doc

[roywei]

Summary

removing sparse documentation for keras-mxnet 2.2.4 release, will add this with followup sparse release

Related Issues

PR Overview

• [ ] This PR requires new unit tests [y/n] (make sure tests are included)
• [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [ ] This PR is backwards compatible [y/n]
• [ ] This PR changes the current API [y/n]