Remove outdated ruby-debug requirement #365

Job	Run time
lint	35s
test (3.3, 7.2, --enable-frozen-string-literal)	1m 33s
test (3.3, 7.1, --enable-frozen-string-literal)	1m 39s
test (3.3, 7.0, --enable-frozen-string-literal)	1m 29s
test (3.3, 6.1, --enable-frozen-string-literal)	1m 17s
test (3.2, 7.2)	1m 23s
test (3.2, 7.1)	1m 31s
test (3.2, 7.0)	1m 23s
test (3.2, 6.1)	1m 22s
test (3.1, 7.1)	1m 18s
test (3.1, 7.0)	1m 22s
test (3.1, 6.1)	1m 23s
test (3.0, 7.0)	1m 26s
test (3.0, 6.1)	1m 17s
test (jruby-9.4.8.0, 7.0)	4m 14s
test (2.7, 7.0)	1m 29s
test (2.7, 6.1)	1m 34s
test (2.7, 6.0)	1m 37s
test (jruby-9.3.15.0, 6.1)	3m 52s
test (2.6, 5.2)	1m 34s
	33m 18s

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