refactor(http1): use saturating_sub instead of manual impl #3113

Job	Run time
Verify hyper.h is up to date	21s
Check Style	9s
semver	28s
Check MSRV	25s
Test C API (FFI)	21s
Test cargo-c support (FFI)	27s
Test stable on ubuntu-latest	51s
Test stable on windows-latest	1m 55s
Test nightly on windows-latest	2m 24s
Test stable on macOS-latest	1m 28s
Test beta on ubuntu-latest	57s
Test beta on windows-latest	2m 0s
Test nightly on macOS-latest	1m 41s
minimal-versions	32s
Test beta on macOS-latest	1m 2s
Test nightly on ubuntu-latest	1m 8s
udeps	22s
features	52s
Test with Miri	2m 28s
Build docs	21s
Check exposed types	23s
CI is green	3s
	20m 38s

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