Remove redundant require_relative

This PR removes a redundant require statement that is already present in the `server.rb`:
https://github.com/modelcontextprotocol/ruby-sdk/blob/89ef9d8/lib/model_context_protocol/server.rb#L5

Author: koic

lib/model_context_protocol.rb

@@ -18,7 +18,6 @@
 require_relative "model_context_protocol/prompt/result"
 require_relative "model_context_protocol/version"
 require_relative "model_context_protocol/configuration"
-require_relative "model_context_protocol/methods"
 
 module ModelContextProtocol
   class << self