Don't search for special .config files

After doing some code-searching, I'm pretty sure this code path is
unused. Finding files that are otherwised unreferenced in the makefiles/
blueprint files is often very surprising behavior, remove it.

Modules should use soong's extra_test_configs property if they need
to use other test configs.

Bug: 396761822
Test: Presubmits
Change-Id: I13b0f4d84882fb2b6a8f0a34d2e586f80327bc56