linux

Remove `core.h` from README #8128

Job	Run time
build (g++-4.9, Release, 11)	2m 18s
build (g++-4.9, Debug, 11)	1m 31s
build (g++-11, Release, 11)	2m 14s
build (g++-11, Debug, 11)	1m 16s
build (clang++-11, Release, 11)	2m 27s
build (g++-11, Debug, 14, sudo apt install g++-11)	1m 23s
build (clang++-3.6, Debug, 11)	1m 36s
build (g++-11, Debug, 20, sudo apt install g++-11)	1m 43s
build (clang++-11, Debug, 17, -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON, sudo apt install clang-11)	1m 43s
build (g++-11, Debug, 17)	1m 26s
build (clang++-11, Debug, 11)	1m 27s
build (clang++-14, Debug, 20)	2m 5s
build (g++-13, Release, 23, -DBUILD_SHARED_LIBS=ON, sudo apt install g++-13)	3m 38s
build (clang++-11, Debug, 17, -stdlib=libc++, sudo apt install clang-11 libc++-11-dev libc++abi-1...	1m 38s
build (clang++-14, Debug, 20, -stdlib=libc++, sudo apt install libc++-14-dev libc++abi-14-dev)	1m 52s
build (clang++-3.6, Release, 11)	1m 46s
	30m 3s

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