remove junk file from the repository root

Sider has not been used by meson for a long time. Also it appears to no
longer exist.
diff --git a/sider.yml b/sider.yml
deleted file mode 100644
index 5c56196..0000000
--- a/sider.yml
+++ /dev/null
@@ -1,7 +0,0 @@
-linter:
-  flake8:
-    version: 3
-    plugins:
-      - flake8-blind-except
-      - flake8-builtins
-      - flake8-bugbear