Remove extra newlines in warning messages #831

Job	Run time
windows-xbuild (32)	36s
unix-testing (ubuntu-20.04, g++, make)	1m 28s
windows-xbuild (64)	44s
windows-testing (32, windows-2019)	3m 41s
unix-testing (ubuntu-20.04, g++, cmake)	1m 32s
windows-testing (32, windows-2022)	4m 11s
unix-testing (ubuntu-20.04, clang++, make)	1m 27s
windows-testing (64, windows-2019)	3m 36s
unix-testing (ubuntu-20.04, clang++, cmake)	1m 25s
windows-testing (64, windows-2022)	4m 29s
unix-testing (ubuntu-22.04, g++, make)	2m 35s
unix-testing (ubuntu-22.04, g++, cmake)	2m 36s
unix-testing (ubuntu-22.04, clang++, make)	1m 40s
unix-testing (ubuntu-22.04, clang++, cmake)	1m 41s
unix-testing (macos-11, clang++, make)	5m 51s
unix-testing (macos-11, clang++, cmake)	5m 56s
unix-testing (macos-12, clang++, make)	6m 4s
unix-testing (macos-12, clang++, cmake)	6m 49s
windows-xtesting (windows-2019, 32)	2m 22s
windows-xtesting (windows-2019, 64)	2m 20s
windows-xtesting (windows-2022, 32)	2m 49s
windows-xtesting (windows-2022, 64)	2m 39s
	1h 6m 31s

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