Remove unused parent_scheme attribute from root tag of editor… (#117)

The `parent_scheme` is set automatically when a custom editor color
scheme is created based on one of the bundled themes, like e.g.
"Darcula" when the IDE uses a dark theme or “Default“ in light mode.
Anyway, the attribute is not required at all and has been removed.

Moreover, to the time of this PR there's no real indicator in the
"IntelliJ IDEA Community Edition" source code [1] that the attribute has
any functionality but only documenting the code base of the custom
theme [2].

Nevertheless, it is possible that the attribute affects the logic how
editor color schemes handle the inheritance of colors from other syntax
definitions or the "Language Defaults". There was an increasing amount
of reported issues where syntax elements were highlighted with colors
from the "Darcula" editor scheme instead of the ones defined by Nord
although the values must have been derived from other syntax
definitions.
This might be caused by the `parent_scheme` attribute which was set to
"Darcula".
Therefore the attribute has now been removed to mitigate such behavior.

Thanks to Tom1206 [3] for the idea of inspecting the attribute.
See GH-115 for more details.

[1]: https://github.com/JetBrains/intellij-community/search?p=2&q=parent_scheme&unscoped_q=parent_scheme
[2]: https://github.com/JetBrains/intellij-community/blob/4491058316bab4162d2ee0a926ac65553b56e6a5/platform/editor-ui-ex/src/com/intellij/openapi/editor/colors/impl/AbstractColorsScheme.java#L311-L313
[3]: https://github.com/Tom1206

Related to GH-115, GH-116

resources/themes/nord.xml

@@ -1,4 +1,4 @@
-<scheme name="Nord" version="142" parent_scheme="Darcula">
+<scheme name="Nord" version="142">
   <metaInfo>
     <property name="created">2016-09-24T21:08:20</property>
     <property name="ide">idea</property>