[gnome-tweak-tool] Remove obsolete mainwindow.py

[Jeremy Bicha <jbicha src gnome org>]

commit ea49baef85057df4a1714500978d0e16b58b3db3
Author: Jeremy Bicha <jbicha ubuntu com>
Date:   Thu Jun 8 23:14:41 2017 -0400

    Remove obsolete mainwindow.py
    
    Not used since
    https://git.gnome.org/browse/gnome-tweak-tool/commit/?id=c144ad

 gtweak/mainwindow.py |   42 ------------------------------------------
 gtweak/meson.build   |    1 -
 2 files changed, 0 insertions(+), 43 deletions(-)
---
diff --git a/gtweak/meson.build b/gtweak/meson.build
index 67d7f58..d9c659e 100644
--- a/gtweak/meson.build
+++ b/gtweak/meson.build
@@ -22,7 +22,6 @@ shell_sources = [
   'gshellwrapper.py',
   'gtksettings.py',
   '__init__.py',
-  'mainwindow.py',
   'tweakmodel.py',
   'tweakview.py',
   'utils.py',