[torchelastic] Remove stale `test_get_default_executable` test (#68609)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68609
The test is stale and tests non-existent method
Test Plan: ci
Reviewed By: kiukchung
Differential Revision: D32540127
fbshipit-source-id: c47b7aed3df6947819efb2f4ad1b7a059c252138
