axi_xbar: Remove redundant documentation

The documentation for `axi_xbar` is now auto generated from the source file. The diagram has been moved to docs:. It is no longer necessary to keep these files.

Author: Wolfgang Rönninger

CHANGELOG.md

@@ -14,6 +14,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
   with the new `PipelineRequest` and `PipelineResponse` `parameter`s), and disable those pipeline
   registers by default.
 - `axi_xbar`: Flatten and update parameter and ports according to (#153). Add inline documentation.
+  - Remove `doc/axi_xbar.md`.
+  - Move `doc/svg/axi_xbar.svg` and `doc/axi_xbar.png` to `docs/`.
 
 ### Fixed