Revert ".travis.yml: Cache Avocado cache"

This reverts commit c1073e4.

The Avocado tests have been removed from Travis a long time ago with
commit c5008c7 ("gitlab: add acceptance testing to system builds"),
so we don't need to cache the avocado files here anymore.

Message-ID: <20240320104144.823425-4-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

.travis.yml

@@ -12,8 +12,6 @@ cache:
   timeout: 1200
   ccache: true
   pip: true
-  directories:
-  - $HOME/avocado/data/cache
 
 
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