sorbet: delete local fixes that have been merged upstream

[SeekingMeaning]

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Upstream merged PRs:

• Fix signature for IO.open. sorbet/sorbet#3722
• chown arguments can be nil. sorbet/sorbet#3729
• Support include FileUtils sorbet/sorbet#3887 (superseded Support include FileUtils. sorbet/sorbet#3730)

[BrewTestBot]

Review period will end on 2021-01-18 at 15:38:25 UTC.

[BrewTestBot]

Review period ended.