revert: impl From<sigaction> for SigAction #1085

Job	Run time
rust_stable	1m 13s
macos	5m 22s
linux_native_builds (x86_64-unknown-linux-gnu)	3m 26s
linux_native_builds (x86_64-unknown-linux-musl)	3m 28s
macos-aarch64	2m 42s
minver	12s
rustfmt	7s
tier3 (x86_64-unknown-dragonfly)	57s
tier3 (x86_64-unknown-openbsd)	53s
tier3 (x86_64-unknown-haiku)	48s
redox	31s
cross (arm-unknown-linux-gnueabi)	1m 40s
cross (armv7-unknown-linux-gnueabihf)	1m 41s
cross (i686-unknown-linux-gnu)	1m 40s
cross (i686-unknown-linux-musl)	1m 35s
cross (mips-unknown-linux-gnu)	1m 34s
cross_compiling (aarch64-linux-android)	29s
cross (mips64-unknown-linux-gnuabi64)	1m 40s
cross_compiling (arm-linux-androideabi)	31s
cross (mips64el-unknown-linux-gnuabi64)	1m 41s
cross_compiling (armv7-linux-androideabi)	33s
cross (mipsel-unknown-linux-gnu)	1m 41s
cross_compiling (i686-linux-android)	31s
cross (powerpc64le-unknown-linux-gnu)	1m 47s
cross_compiling (x86_64-linux-android)	30s
cross_compiling (arm-unknown-linux-musleabi)	31s
cross_compiling (x86_64-unknown-fuchsia)	27s
cross_compiling (x86_64-unknown-illumos)	27s
cross_compiling (aarch64-apple-ios, true)	52s
cross_compiling (powerpc64-unknown-linux-gnu)	37s
cross_compiling (s390x-unknown-linux-gnu)	36s
cross_compiling (x86_64-unknown-linux-gnux32)	37s
cross_compiling (x86_64-unknown-netbsd)	31s
	41m 50s

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