mqtt: do not own settings #611

Job	Run time
documentation	24s
audit	7s
compile (stable, thumbv7em-none-eabihf)	47s
examples (mqtt)	56s
compile (1.65.0, thumbv7em-none-eabihf)	2m 2s
style	27s
examples (readback)	38s
test (stable)	1m 0s
test (stable, --all-features)	48s
test (stable, --no-default-features)	32s
test (stable, --no-default-features --features json-core)	37s
test (1.65.0)	1m 53s
test (1.65.0, --all-features)	1m 46s
test (1.65.0, --no-default-features)	1m 33s
test (1.65.0, --no-default-features --features json-core)	1m 30s
	15m 0s

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