Remove unused codes

[willnet]

Now that we support Ruby 2.7 and above, we can remove conditions with versions lower than that.

[zzak]

Found this too:

openapi_parser/lib/openapi_parser/schema_validator/string_validator.rb

Lines 58 to 61 in 385a9ab

	# match? method is good performance.
	# So when we drop ruby 2.3 support we use match? method because this method add ruby 2.4
	#return [value, nil] if value.match?(URI::MailTo::EMAIL_REGEXP)
	return [value, nil] if value.match(URI::MailTo::EMAIL_REGEXP)

[willnet]

@zzak Thank you for pointing it out 👍