JDWP: remove thread-related tests for Count modifier

These tests are flaky because other (internal) threads can start/end
in the middle of the test causing its failure. Since we can't ensure
no other thread will start/end in the middle of the test, it's better
to remove them to avoid random failures.

Bug: 18768319

(cherry picked from commit 16838423)

Change-Id: I513b13487fc7e89676e9e4ec3e17e7519f7bb205