refactor: remove package of react-g

[zhongyunWan]

remove package of react-g，because react-g simply won't run in a React 19 project.

[petercat-assistant]

Walkthrough

This pull request removes the react-g package from the g6-extension-react module due to compatibility issues with React 19. The changes involve deleting related imports, exports, and demo files associated with GNode and its components.

Changes

Files	Summary
packages/g6-extension-react/README.md	Removed GNode import and registration.
packages/g6-extension-react/__tests__/demos/g-node.tsx packages/g6-extension-react/src/elements/g/node.tsx packages/site/examples/element/custom-node/demo/react-g.jsx	Deleted files related to GNode component and demos.
packages/g6-extension-react/__tests__/demos/index.tsx	Removed export of g-node demo.
packages/g6-extension-react/src/elements/g/index.ts	Removed GNode export.
packages/g6-extension-react/src/elements/g/shapes.ts	Changed import source from @antv/react-g to @antv/g.
packages/g6-extension-react/src/index.ts	Removed GNode and its style props from exports.
packages/g6-extension-react/package.json	Updated dependencies and configurations.