Cleanup humble branch (#545)

* Update branch references for Humble in main README

* Update build badges to reference scheduled builds

We do the same for foxy and galactic which do not live on the main branch.

* Updated CI workflows to refer to humble branch and removed schedules

Scheduled runs need to live on the main branch. We should be able to remove
them from the distro-specific branch.

* Remove repos files for other distros

* Remove faulty line from CHANGELOG

Since rst usually requires double backticks for code, our rst check will fail, otherwise.

.github/workflows/humble-binary-build.yml

@@ -6,9 +6,6 @@ on:
   push:
     branches:
       - humble
-  schedule:
-    # Run every morning to detect flakiness and broken dependencies
-    - cron: '13 4 * * *'
 
 jobs:
   humble_binary:

.github/workflows/humble-semi-binary-build.yml

@@ -6,9 +6,6 @@ on:
   push:
     branches:
       - humble
-  schedule:
-    # Run every morning to detect flakiness and broken dependencies
-    - cron: '03 5 * * *'
 
 jobs:
   humble_semi_binary:

.github/workflows/humble-source-build.yml

@@ -2,10 +2,7 @@ name: Humble Source Build
 on:
   push:
     branches:
-      - main
-  schedule:
-    # Run every morning to detect flakiness and broken dependencies
-    - cron: '43 1 * * *'
+      - humble
 
 jobs:
   humble_source: