Remove Wiki* RPC handlers - Part 1

Wiki feature group is a well-known feature of GitLab. Underlying, all Wiki data are organized as Git repository. In prior versions, we created dedicated RPCs to handle Wiki-related requests. Those RPCs are then handled by Gitaly Ruby sidecar (powered by Gollum and Rugged gem).

In an attempt to remove the ruby sidecar completely, we migrated clients to use normal repository to handle Wiki-specific logic. This migration finished in %15.5. It's time to wipe corresponding handlers in Gitaly.

This MR does not touch the code paths in ruby sidecar. I'll clean it in the next MR.

At the time of writing, it's safe to remove these RPC handlers:

• ✅ All client codes to Wiki* RPCs were removed in gitlab!100821 (merged).
• ✅ The above MR should be included in %15.5. This MR targets %15.6. This prevents a backward-compatibility issue mentioned in &2381 (comment 1129451097)
• ✅ There is no traffic to them on both Staging and Production environments:
  • WikiFindPage
  • WikiGetAllPages
  • WikiListPages
  • WikiUpdatePage
  • WikiWritePage

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Testing the gRPC server by trigger an RPC in Rails console raises GRPC::Unimplemented excepion:

GRPC::Unimplemented: 12:unknown service gitaly.WikiService. debug_error_string:{"created":"@1666165725.539074000","description":"Error received from peer unix:gitaly.socket","file":"src/core/lib/surface/call.cc","file_line":1064,"grpc_message":"unknown service gitaly.WikiService","grpc_status":12}
from ~/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/grpc-1.42.0-x86_64-darwin/src/ruby/lib/grpc/generic/active_call.rb:29:in `check_status'