[UX] Fix redundant disk tier information when default tier is used

[cblmemo]

Currently, Azure clusters launched without disk_tier specified will have a disk_tier in its Resources in the sky status command. This PR get rid of the redundant information.

$ sky launch --cloud azure
Clusters
NAME                          LAUNCHED        RESOURCES                                    STATUS   AUTOSTOP  COMMAND                         
cluster-name                  a few secs ago  1x Azure(Standard_D8s_v5, disk_tier=medium)  UP       -         sky launch --cloud azure ...   

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

Thanks for the quick fix @cblmemo! LGTM. Just a small comment for readability.