dt-bindings: net: dsa: remove obsolete dsa.txt

dsa.txt has been a redirect to dsa.yaml since commit bce58590d1bd
("dt-bindings: net: dsa: Add DSA yaml binding") introduced the .yaml
schema. The .yaml has the same filename in the same directory, making
this redirect unnecessary for discoverability.

Two files still reference dsa.txt, forcing readers through an extra
hop to reach the .yaml. The stub has not been touched since August
2020. Update references in lan9303.txt and
Documentation/networking/dsa/dsa.rst to point directly to dsa.yaml
and remove the stub.

Signed-off-by: Akash Sukhavasi <akash.sukhavasi@gmail.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260603-b4-remove-redirect-stubs-v2-3-c8c19876ab64@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>