Remove temporary fix for sorbet runtime regression

sorbet/sorbet#4074 has been merged which solves
the Ruby back compatibility problem. We don't have to keep pinning
`sorbet-runtime` to older versions anymore.

spec/support/repo/Gemfile

@@ -10,9 +10,6 @@ gem("baz", path: "../gems/baz")
 gem("qux", path: "../gems/qux", require: false)
 gem("tapioca", path: "../../../")
 
-# Temp fix for https://github.com/sorbet/sorbet/pull/4074
-gem("sorbet-runtime", "< 0.5.6331")
-
 gem("psych")
 # `extras` gem is causing problems by modifying
 # `Shellwords` functionaly. Depending on it to