Remove the YAML tidy code (#30004)

This code was used to test buildbox_steps.yml, but Servo no longer uses
buildbot, so this code is essentially unused. In addition, YAML +
Cython 3 is causing issues on the CI.

Fixes #30003

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

python/tidy/tests/duplicate_keys_buildbot_steps.yml

@@ -1,7 +0,0 @@
----
-duplicate_yaml_key:
-    - value1
-other_key:
-    - value2
-duplicate_yaml_key:
-    - value3

python/tidy/tests/non_list_mapping_buildbot_steps.yml

@@ -1,2 +0,0 @@
----
-non-list-key: "string string"

python/tidy/tests/non_string_list_buildbot_steps.yml

@@ -1,7 +0,0 @@
----
-# This is a buildbot_steps.yml file that should break linting becasue it is not a
-# mapping to a list of strings
-mapping_key:
-    - - list_of_list
-      - sublist_item1
-      - sublist_item2