Delete TextAncestor compatibility shim

The shim file `packages/react-native/Libraries/Text/TextAncestor.js`
was kept solely to support react-strict-dom's deep import. The
file's own comment carried a TODO from @huntie to delete it once
the cross-repo reference was fixed.

react-strict-dom has now switched to the public
`unstable_TextAncestorContext` API
(react/react-strict-dom#477), exposed in #52368.

No internal React Native consumers reference this shim — both
`packages/react-native/index.js` and `index.js.flow` import
`./Libraries/Text/TextAncestorContext` directly.

## Changelog:

[INTERNAL] - Delete TextAncestor compatibility shim

Author: Yamin Yassin