readme: Remove source layout section

This section provides limited use since Github as well as modern IDEs
have good file browsing built in. The source layout section consumes
valuable visual space on the Github landing page and distracts from
other important information.

README.md

@@ -26,34 +26,6 @@ This work is dual-licensed under the Eclipse Public License v2.0 and Eclipse Dis
 
 [![Build](https://github.com/eclipse/wakaama/actions/workflows/build_and_test.yaml/badge.svg)](https://github.com/eclipse/wakaama/actions/workflows/build_and_test.yaml)
 
-## Source Layout
-
-    -+- core                   (the LWM2M engine)
-     |
-     +- coap                   (CoAP stack adaptation)
-     |    |
-     |    +- er-coap-13        (Modified Erbium's CoAP engine from
-     |                          https://web.archive.org/web/20180316172739/http://people.inf.ethz.ch/mkovatsc/erbium.php)
-     |
-     +- data                   (data formats serialization/deserialization)
-     |
-     +- tests                  (test cases)
-     |    |
-     |    +- integration       (pytest based integration tests implementing the OMA-ETS-LightweightM2M-V1_1-20190912-D specification
-     |                          https://www.openmobilealliance.org/release/LightweightM2M/ETS/OMA-ETS-LightweightM2M-V1_1-20190912-D.pdf)
-     +- examples
-          |
-          +- bootstrap_server  (a command-line LWM2M bootstrap server)
-          |
-          +- client            (a command-line LWM2M client with several test objects)
-          |
-          +- lightclient       (a very simple command-line LWM2M client with several test objects)
-          |
-          +- server            (a command-line LWM2M server)
-          |
-          +- shared            (utility functions for connection handling and command-
-                                line interface)
-
 ## Checking out the code
 
 ### Using Wakaama as library