Fix CMake duplication, update Readme

[kstribrnAmzn]

Description of changes

Remove duplicate and unnecessary CMake line. Update README to add notice that default Root CA path is relative to binary execution directory.

Issue

https://github.com/aws/aws-iot-device-sdk-embedded-C/issues/1880

Testing

Built mqtt_mutual_auth demo without an issue. Executed demo successfully.

Commands to build

cmake -S . -B build -DAWS_IOT_ENDPOINT="myendpoint-ats.iot.us-east-1.amazonaws.com" -DCLIENT_CERT_PATH="/home/kstribrn/cool-certificate.pem.crt" -DCLIENT_PRIVATE_KEY_PATH="/home/kstribrn/my-privatekey.pem.key"
cd build
make mqtt_demo_mutual_auth
cd bin
./mqtt_demo_mutual_auth

Output

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:680] Establishing a TLS session to myendpoint-ats.iot.us-east-1.amazonaws.com:8883.
[INFO] [MQTT] [core_mqtt.c:2740] MQTT connection established with the broker.
[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1173] MQTT connection successfully established with broker.

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1683] A clean MQTT connection is established. Cleaning up all the stored outgoing publishes.

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1418] Subscribing to the MQTT topic testclient/example/topic.
[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1237] SUBSCRIBE sent for topic testclient/example/topic to broker.

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1051] Subscribed to the topic testclient/example/topic. with maximum QoS 1.

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1457] Sending Publish to the MQTT topic testclient/example/topic.
[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1335] PUBLISH sent for topic testclient/example/topic to broker with packet ID 2.

[INFO] [MQTT] [core_mqtt.c:1540] Ack packet deserialized with result: MQTTSuccess.
[INFO] [MQTT] [core_mqtt.c:1557] State record updated. New state=MQTTPublishDone.
[INFO] [DEMO] [mqtt_demo_mutual_auth.c:1082] PUBACK received for packet id 2.

[INFO] [DEMO] [mqtt_demo_mutual_auth.c:792] Cleaned up outgoing publish packet with packet id 2.
...

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.