CNDB-13624: Silence snakeyaml warnings in test-cdc by removing duplicate cdc_enabled key in base yaml (#1680)

### What is the issue
#1563 upgraded our version of snakeyaml. This new version logs when
duplicate keys are found. The test-cdc target concatenates a
CDC-specific yaml to the base yaml. Both contain entries for the
cdc_enabled key, causing tests that load this yaml to log the warning.
This causes CDC CI failures.

### What does this PR fix and why was it fixed
This PR removes the cdc_enabled entry in the base yaml, as it's the same
as the default. This is consistent with how we handle other default keys
that might be duplicated across multiple yaml fragments in the test
commands.

Author: jkni

test/conf/cassandra.yaml

@@ -12,7 +12,6 @@ commitlog_directory: build/test/cassandra/commitlog
 # commitlog_compression:
 # - class_name: LZ4Compressor
 cdc_raw_directory: build/test/cassandra/cdc_raw
-cdc_enabled: false
 hints_directory: build/test/cassandra/hints
 metadata_directory: build/test/cassandra/metadata
 partitioner: org.apache.cassandra.dht.ByteOrderedPartitioner