pkg/covermerger: remove excessive logs #14896

Job	Run time
build	10m 56s
aux	4m 52s
dashboard	3m 48s
arch (presubmit_arch_linux)	10m 50s
arch (presubmit_arch_freebsd)	6m 36s
gvisor	4m 12s
arch (presubmit_arch_netbsd)	4m 28s
old	6m 43s
arch (presubmit_arch_openbsd)	4m 20s
race	8m 58s
arch (presubmit_arch_darwin)	4m 27s
race_dashboard	4m 22s
arch (presubmit_arch_windows)	3m 24s
arch (presubmit_arch_executor)	3m 13s
	1h 21m 9s

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