[ruby] Combine *-sequel frameworks with their main frameworks

[p8]

rack-sequel and sinatra-sequel only use a different ORM (sequel) than rack and sinatra. There is no need to test /json and /plaintext for rack-sequel and sinatra-sequel, as these calls don't use the ORM.

For sinatra-sequel the /json and /plaintext calls were already removed from the benchmark config in 1f94f389dcc30f1795b23953ad152560d5433f9f. This change does the same for rack-sequel.

To still see these results in the combined table, the framework is set to rack and sinatra.

[p8]

~Hmm, ok I'll probably need to merge the directories instead.~ Nevermind, this works now as expected similar to Ruby/agoo: https://github.com/TechEmpower/FrameworkBenchmarks/blob/77513b8d34ce60860d47d9f95e9c5035d605dca9/frameworks/Ruby/agoo/benchmark_config.json#L15