Remove stale Hugo submodule references

The doc/ Hugo site was migrated to ossum.tech. The three submodule
entries in .gitmodules (hugo-theme-learn, redislabs-docs,
hugo-theme-docdock) pointed to themes no longer used and caused
warnings on pull. The submodule paths were already removed from
the tree; this removes the leftover .gitmodules file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: reid-spencer