remove unused abbrev dependency #50

Job	Run time
test (ubuntu-latest, head)	1m 6s
test (ubuntu-latest, 3.3)	1m 12s
test (ubuntu-latest, 3.2)	1m 12s
test (ubuntu-latest, 3.1)	1m 14s
test (ubuntu-latest, 3.0)	1m 19s
test (ubuntu-latest, jruby)	39s
test (ubuntu-latest, jruby-head)	36s
test (ubuntu-latest, truffleruby)	1m 49s
test (ubuntu-latest, truffleruby-head)	2m 0s
test (ubuntu-latest, truffleruby+graalvm)	4m 32s
test (windows-latest, head)	1m 56s
test (windows-latest, 3.3)	2m 46s
test (windows-latest, mingw)	1m 51s
test (windows-latest, mswin)	1m 27s
test (windows-latest, ucrt)	1m 41s
test (macos-latest, head)	2m 1s
test (macos-latest, 3.3)	2m 18s
	29m 39s

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