v5 cleanup: Remove the obsolete context.IsPostback setter #3280

Job	Run time
.NET unit tests (ubuntu-latest)	2m 47s
Build published projects without warnings (Release)	2m 55s
.NET unit tests (windows-2022)	7m 7s
Build published projects without warnings (Debug)	2m 46s
.NET unit tests (macOS-latest)	2m 53s
Build all projects without errors	10m 48s
JS unit tests	1m 3s
UI tests (chrome, ubuntu-latest, Production, Default)	15m 5s
UI tests (chrome, windows-2022, Development, Default)	29m 4s
UI tests (firefox, ubuntu-latest, Development, Default)	19m 25s
UI tests (firefox, ubuntu-latest, Production, Default)	18m 8s
UI tests (firefox, ubuntu-latest, Development, ExperimentalFeaturesTests)	19m 50s
UI tests (chrome, ubuntu-latest, Production, ExperimentalFeaturesTests)	15m 54s
	2h 27m 45s

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