[gnome-builder] build: remove old docs

[Christian Hergert <chergert src gnome org>]

commit fd6859b7b7575d115beed9bf78302f091a091589
Author: Christian Hergert <chergert redhat com>
Date:   Tue Feb 23 22:44:37 2021 -0800

    build: remove old docs

 doc/help/howto/index.rst            |  1 -
 doc/help/howto/rust_integration.rst | 20 --------------------
 2 files changed, 21 deletions(-)
---
diff --git a/doc/help/howto/index.rst b/doc/help/howto/index.rst
index ebe0a3518..a42cdc1ca 100644
--- a/doc/help/howto/index.rst
+++ b/doc/help/howto/index.rst
@@ -16,5 +16,4 @@ Contents
    search_and_replace
    jump_to_definition
    search_for_symbol
-   rust_integration
    sysroot