bpo-35610: IDLE - Replace .context_use_ps1 with .prompt_last_line (GH-11307)

Changes in bpo- 31858 made the less informative 'context_use_ps1' redundant.
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index 2de4265..6e0707d 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -881,7 +881,7 @@
         self.usetabs = True
         # indentwidth must be 8 when using tabs.  See note in EditorWindow:
         self.indentwidth = 8
-        self.context_use_ps1 = True
+
         self.sys_ps1 = sys.ps1 if hasattr(sys, 'ps1') else '>>> '
         self.prompt_last_line = self.sys_ps1.split('\n')[-1]
         self.prompt = self.sys_ps1  # Changes when debug active