fix: remove deprecated `Application::getTwitchAbstract` #10121

Job	Run time
Build macos-13, Qt 5.15.2 (LTO:false, crashpad:on)	9m 3s
Build windows-latest, Qt 6.7.1 (LTO:false, crashpad:on)	14m 28s
Build windows-latest, Qt 5.15.2 (LTO:false, crashpad:off)	11m 30s
Build Ubuntu in Docker (ubuntu-20.04, ghcr.io/chatterino/chatterino2-build-ubuntu-20.04:latest, 6...	11m 6s
Build Ubuntu in Docker (ubuntu-22.04, ghcr.io/chatterino/chatterino2-build-ubuntu-22.04:latest, 6...	10m 15s
create-release	0s
	56m 22s

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