ci(system-tests): Remove sed hacks for android module exclusion (#5682)

Remove the sed-based Android module exclusion from settings.gradle.kts
and build.gradle.kts in the backend system tests workflow. This is
unnecessary because org.gradle.configureondemand=true ensures Gradle
only configures projects needed for the requested tasks, and the test
runner already builds targeted sample modules rather than the whole
project.

This mirrors #5397, which removed the same sed hacks from the Spring
Boot matrix workflows.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>