Remove the obsolete version of Explain Code feature

Overview

Explain Code feature was migrated to open Duo Chat: Introduced chat capabilities to Explain Code (!117674 - merged). As a result, Explain Code Duo Chat tool was introduced: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/llm/chain/tools/explain_code/executor.rb. It means that Explain Code service along with the related classes can be removed: https://gitlab.com/gitlab-org/gitlab/-/blob/fe60e1b6236c7e4837f201b0750bfe9f09d5c4d3/ee/lib/gitlab/llm/utils/ai_features_catalogue.rb#L45-53.

Proposal

• Remove explain code feature: https://gitlab.com/gitlab-org/gitlab/-/blob/fe60e1b6236c7e4837f201b0750bfe9f09d5c4d3/ee/lib/gitlab/llm/utils/ai_features_catalogue.rb#L45-53
• Remove ::Gitlab::Llm::VertexAi::Completions::ExplainCode
• Remove ::Gitlab::Llm::VertexAi::Templates::ExplainCode
• Remove ::Llm::ExplainCodeService
• Remove explain_code_chat feature flag