Switch sample/ to the plugin's iOS wiring and remove the library-side duplicates (#90 follow-up) #321

Follow-up of #90 (landed in #320). sample/build.gradle.kts still Syncs Python.xcframework and adds the -framework Python linkerOpts itself, and python-multiplatform/build.gradle.kts still registers its own stageIosPythonHome* tasks and its iOS download. Switch sample/ to the plugin path (preparePythonXcframework, installPythonXcodeScript) and remove what only duplicates it, keeping what the library's own tests and cinterop need. Completion: sample iOS simulator app builds and runs through the plugin path; no duplicated task left (list what stays and why).