Deduplicate merged GPU quantile sketch entries #3788

Job	Run time
Configure variables for CI	9s
Build 32-bit (i386)	2m 50s
Test installing XGBoost lib + building the C API demo	8m 55s
Test Google C++ unittest (CPU Non-OMP)	9m 49s
Build docs for JVM packages	12m 32s
Build docs for the R package	10m 21s
Trigger Read The Docs build	10s
	44m 46s

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