Removed needless NullWriter class #493

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.6 / ubuntu-latest)	37s
test (2.6 / macos-latest)	41s
test (2.6 / windows-latest)	4m 26s
test (2.7 / ubuntu-latest)	34s
test (2.7 / macos-latest)	39s
test (2.7 / windows-latest)	2m 47s
test (3.0 / ubuntu-latest)	22s
test (3.0 / macos-latest)	26s
test (3.0 / windows-latest)	1m 32s
test (3.1 / ubuntu-latest)	22s
test (3.1 / macos-latest)	26s
test (3.1 / windows-latest)	1m 35s
test (3.2 / ubuntu-latest)	22s
test (3.2 / macos-latest)	25s
test (3.2 / windows-latest)	1m 6s
test (3.3 / ubuntu-latest)	21s
test (3.3 / macos-latest)	27s
test (3.3 / windows-latest)	1m 54s
test (head / ubuntu-latest)	20s
test (head / macos-latest)	25s
test (head / windows-latest)	1m 17s
	21m 7s

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