[lldb/dotest] Remove the "exclusive test subdir" concept

Summary:
This was added in 2018 (r339929), when we were still using the
hand-rolled test runner.

It does not seem to be relevant anymore. In fact as far as I can tell,
it's a big no-op now as the exclusive_test_subdir variable is never set.

Reviewers: vsk, JDevlieghere

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74551