ci: remove duplicate gradle dependabot entry for /app

[kinyoklion]

Summary

The dependabot.yml had two gradle ecosystem entries — one for / and one for /app. Since the root / entry scans the entire Gradle project including subprojects, both entries discovered the same launchdarkly-android-client-sdk dependency in app/build.gradle, producing duplicate PRs (#29 and #30) for the same version bump.

Removes the redundant /app entry so only the root-level entry remains.

Link to Devin session: https://app.devin.ai/sessions/abb872ed8f024de08652a0e4bd201f43
Requested by: @kinyoklion

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