ci: Remove Ubuntu 16 #189

Job	Run time
ubuntu-latest BUILD=static CC=gcc-7 CXX=g++-7 AUTOTOOLS=no	13s
Windows MinGW build (shared, x64)	5m 21s
Windows MSVC build (Release, Win64)	4m 22s
ubuntu-latest BUILD=shared CC=gcc CXX=g++ AUTOTOOLS=yes	23s
Windows MinGW build (static, x64)	4m 49s
Windows MSVC build (Debug, Win64)	4m 48s
ubuntu-latest BUILD=static CC=gcc CXX=g++ AUTOTOOLS=yes	37s
Windows MinGW build (shared, x86)	4m 9s
Windows MSVC build (Release, Win32)	4m 59s
ubuntu-latest BUILD=shared CC=gcc CXX=g++ AUTOTOOLS=no	48s
Windows MinGW build (static, x86)	3m 52s
Windows MSVC build (Debug, Win32)	3m 8s
ubuntu-latest BUILD=static CC=gcc CXX=g++ AUTOTOOLS=no	50s
ubuntu-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=yes	27s
ubuntu-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=yes	26s
ubuntu-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=no	1m 18s
ubuntu-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=no	1m 16s
macOS-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=yes	1m 20s
macOS-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=yes	1m 3s
macOS-latest BUILD=shared CC=clang CXX=clang++ AUTOTOOLS=no	1m 37s
macOS-latest BUILD=static CC=clang CXX=clang++ AUTOTOOLS=no	1m 9s
	46m 55s

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