refactor(plugin): remove the unused flatDir injection

[IronTony]

Removes the config plugin step that injected a flatDir block into the consumer's root android/build.gradle. It had no effect, and on current Expo templates it never ran. This is the last change planned before publishing 2.0.0.

Why

• The module's android/build.gradle has referenced the stripped sdk-pdf AAR by file path since 1.0.4 (implementation files("$projectDir/libs/sdk-pdf-2.1.7-stripped.aar")). That change was made because Gradle 9 stopped honoring subproject-scoped flatDir. Nothing reads a flatDir repository anymore.
• The injection's regex allowed one level of braces inside allprojects. The Expo template's maven { url 'https://www.jitpack.io' } inside repositories is two levels, so on SDK 55 to 57 the block was never written and expo prebuild said nothing.
• Kotlin Script projects got a warning with manual flatDir instructions they did not need.

Changes

• plugin/src/index.ts: deletes withDocuSignAndroidStrippedAarFlatDir, FLAT_DIR_MARKER, the .kts warning and the call in withDocuSign. Two comments now describe the file-path reference instead.
• plugin/src/index.test.ts: a new test asserts the only build.gradle change the plugin makes is the DocuSign repository line.
• README.md: the Glide workaround section no longer says a flatDir injection picks up the AAR.
• CHANGELOG.md: a 2.0.0 Fixes entry.

Verification

• The new test fails with the flatDir mod registered and passes without it. It runs the real projectBuildGradle mod chain.
• Plugin tsc, build, lint, examples type-check and all 101 Jest tests pass.
• The package was packed from this branch and installed in an Expo SDK 57 app:
  • Prebuild produced a root build.gradle byte-identical to the previous package's output, with one DocuSign maven line and no flatDir block.
  • The Android release build passed checkReleaseDuplicateClasses.
• Pre-push review returned SHIP on 2fd1e06. It found no remaining reader of a flatDir repository in android/, plugin/, scripts/, docs/ or the README, and confirmed git log -S dates the file-path reference to the 1.0.4 release commit.

Notes

• Apps on a template where the old regex did match may still have the react-native-docusign-stripped-aar-flatdir block in a checked-in android/build.gradle. It stays inert and is safe to delete. expo prebuild --clean removes it.
• Coverage for plugin/src/index.ts reads 43%, down from 49%, because the deleted mod was code the tests exercised. The untested part is the same as before: the AAR download, strip and cache paths.