Remove ASSERT from EphTombstoneTest.ConcurrentPurge

An ASSERT was added to EphTombstoneTest.ConcurrentPurge to remove a compiler warning for a used variable purged. This was incorrect to add as purged is not guaranteed to be > 0, as the test isn't testing if we purge items just that we're thread safe. So to fix the test failing and the compiler warning, remove the variable and ASSERT_GT().

Error signature

Failure of GoogleTest 'EphTombstoneTest.ConcurrentPurge':

<pre>

[ RUN      ] EphTombstoneTest.ConcurrentPurge

../kv_engine/engines/ep/tests/module_tests/ephemeral_vb_test.cc(827): error: Expected: (purged) > (0), actual: 0 vs 0

[  FAILED  ] EphTombstoneTest.ConcurrentPurge (X ms)

EphTombstoneTest.ConcurrentPurge

</pre>

Details
10 instances of this failure (23.3% of sampled failures):

• Time: 2022-04-04 08:25:11, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44884/, patch: https://review.couchbase.org/c/kv_engine/+/173205
• Time: 2022-04-04 08:25:11, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44883/, patch: https://review.couchbase.org/c/kv_engine/+/173204
• Time: 2022-04-01 17:18:15, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44881/, patch: https://review.couchbase.org/c/kv_engine/+/173149
• Time: 2022-04-01 15:35:14, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44878/, patch: https://review.couchbase.org/c/kv_engine/+/173044
• Time: 2022-03-31 12:27:53, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44819/, patch: https://review.couchbase.org/c/kv_engine/+/172593
• Time: 2022-04-01 09:31:10, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44857/, patch: https://review.couchbase.org/c/kv_engine/+/172336
• Time: 2022-03-31 12:27:53, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44820/, patch: https://review.couchbase.org/c/kv_engine/+/172594
• Time: 2022-03-31 13:23:24, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44826/, patch: https://review.couchbase.org/c/kv_engine/+/172781
• Time: 2022-03-31 09:55:30, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44804/, patch: https://review.couchbase.org/c/kv_engine/+/171271
• Time: 2022-03-31 08:22:09, Jenkins job: http://cv.jenkins.couchbase.com/job/kv_engine-windows-master/44793/, patch: https://review.couchbase.org/c/kv_engine/+/171792