Revert "Small fix for slice before main window visible."

This reverts commit 35d6aad6cd7e8a16fbb829dcf8f0b70db12d0028.
This is causing a very weird crash when autoslice is enabled:
2021-01-28 12:12:35,379 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [66]: An uncaught error has occurred!
2021-01-28 12:12:35,379 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]: Traceback (most recent call last):
2021-01-28 12:12:35,379 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:   File "/home/trin/Gedeeld/Projects/Cura/plugins/CuraEngineBackend/CuraEngineBackend.py", line 270, in slice
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:     self._createSnapshot()
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:   File "/home/trin/Gedeeld/Projects/Cura/cura/Utils/Threading.py", line 31, in _call_on_qt_thread_wrapper
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:     return func(*args, **kwargs)
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:   File "/home/trin/Gedeeld/Projects/Cura/plugins/CuraEngineBackend/CuraEngineBackend.py", line 254, in _createSnapshot
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]:     if not CuraApplication.getInstance().isVisible:
2021-01-28 12:12:35,380 - CRITICAL - [MainThread] cura.CrashHandler.__init__ [69]: AttributeError: 'CuraApplication' object has no attribute 'isVisible'

The property exists all right. It's in the dir() of the CuraApplication instance. PyCharm's debug mode also gives a traceback when trying to evaluate the property. It clearly thinks it's there, but then it's not or something. Very weird. We need to take a better look at this than this quick fix.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -250,9 +250,6 @@ class CuraEngineBackend(QObject, Backend):
     @call_on_qt_thread  # must be called from the main thread because of OpenGL
     def _createSnapshot(self) -> None:
         self._snapshot = None
-        if not CuraApplication.getInstance().isVisible():
-            Logger.log("w", "Attempt to create snapshot before complete initialization.")
-            return
         Logger.log("i", "Creating thumbnail image (just before slice)...")
         try:
             self._snapshot = Snapshot.snapshot(width = 300, height = 300)