Remove dead code for Rails 5.x, 6.x, and < 6.1.4 #437

Job	Run time
minitest (3.0, rails_6_1_4, vips)	32s
minitest (3.0, rails_6_1_4, mini_magick)	37s
minitest (3.0, rails_7_0, vips)	31s
minitest (3.0, rails_7_0, mini_magick)	45s
minitest (3.0, rails_7_1, vips)	31s
minitest (3.0, rails_7_1, mini_magick)	35s
minitest (3.0, rails_next, vips)	36s
minitest (3.0, rails_next, mini_magick)	57s
minitest (3.1, rails_7_0, vips)	32s
minitest (3.1, rails_7_0, mini_magick)	38s
minitest (3.1, rails_7_1, vips)	51s
minitest (3.1, rails_7_1, mini_magick)	54s
minitest (3.1, rails_next, vips)	44s
minitest (3.1, rails_next, mini_magick)	43s
minitest (3.2, rails_7_0, vips)	46s
minitest (3.2, rails_7_0, mini_magick)	43s
minitest (3.2, rails_7_1, vips)	29s
minitest (3.2, rails_7_1, mini_magick)	47s
minitest (3.2, rails_next, vips)	44s
minitest (3.2, rails_next, mini_magick)	44s
	13m 39s

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