Eslint config: upgrade eslint-plugin-react to version 7.26.0

This change updates `eslint-plugin-react` to the latest version 7.26.0
and workarounds `fbt` oddities discovered in the previous version (jsx-eslint/eslint-plugin-react#3080).
I added 2 fixtures to make sure `no-unused-vars` rule works as expected
with FBT.

Additionally, it removes unused suppression comments that were also fixed
in previous React plugin version (jsx-eslint/eslint-plugin-react#3063).

Changelog: https://github.com/yannickcr/eslint-plugin-react/blob/eeb0144f14aa972f533e2ef0b094f6742d63c492/CHANGELOG.md#7260---20210920

adeira-source-id: 97d82ba734142c6ba2e1c37e3ab006477659b77c

src/ShipitConfig.js

@@ -67,14 +67,8 @@ export default class ShipitConfig {
   getDefaultShipitFilter(): ChangesetFilter {
     return (changeset: Changeset) => {
       const ch1 = addTrackingData(changeset);
-      // React Eslint plugin bug: https://github.com/yannickcr/eslint-plugin-react/issues/3063
-      // eslint-disable-next-line react/no-this-in-sfc
       const ch2 = stripExceptDirectories(ch1, this.getSourceRoots());
-      // React Eslint plugin bug: https://github.com/yannickcr/eslint-plugin-react/issues/3063
-      // eslint-disable-next-line react/no-this-in-sfc
       const ch3 = moveDirectories(ch2, this.directoryMapping);
-      // React Eslint plugin bug: https://github.com/yannickcr/eslint-plugin-react/issues/3063
-      // eslint-disable-next-line react/no-this-in-sfc
       const ch4 = stripPaths(ch3, this.strippedFiles);
 
       // First we comment out lines marked with `@x-shipit-disable`.