Stop building and testing mingw cross-toolchain #3736

Job	Run time
rustfmt	10s
dist (ubuntu-20.04, x86_64-unknown-linux-gnu)	1m 42s
test (ubuntu-latest, x86_64-unknown-linux-gnu)	7m 2s
dist (macos-latest, x86_64-apple-darwin)	4m 11s
test (macos-latest, x86_64-apple-darwin)	19m 12s
dist (macos-latest, aarch64-apple-darwin)	1m 30s
test_llvm	3m 41s
test (ubuntu-latest, aarch64-unknown-linux-gnu, gcc-aarch64-linux-gnu qemu-user)	4m 25s
test (macos-latest, aarch64-apple-darwin)	5m 40s
dist (windows-latest, x86_64-pc-windows-msvc)	2m 17s
dist (windows-latest, x86_64-pc-windows-gnu)	2m 40s
bench	7m 46s
test (ubuntu-latest, s390x-unknown-linux-gnu, gcc-s390x-linux-gnu qemu-user)	4m 54s
test (ubuntu-latest, riscv64gc-unknown-linux-gnu, gcc-riscv64-linux-gnu qemu-user)	4m 21s
test (windows-latest, x86_64-pc-windows-msvc)	16m 3s
test (windows-latest, x86_64-pc-windows-gnu)	14m 6s
release	17s
	1h 39m 57s

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