only remove attribute if it exists

Attempt to fix #2264 which only rears its head during history replay (on
linux).

news/fix_history_replay.rst

@@ -0,0 +1,14 @@
+**Added:** None
+
+**Changed:** None
+
+**Deprecated:** None
+
+**Removed:** None
+
+**Fixed:**
+
+* ``history replay`` no longer barfs on ``style_name`` when setting up the
+  environment
+
+**Security:** None

xonsh/tools.py

@@ -1576,6 +1576,7 @@ def intensify_colors_on_win_setter(enable):
     """
     enable = to_bool(enable)
     if hasattr(builtins, '__xonsh_shell__'):
+        if hasattr(builtins.__xonsh_shell__.shell.styler, 'style_name'):
             delattr(builtins.__xonsh_shell__.shell.styler, 'style_name')
     return enable