chore: refactor codebase by removing unused linters

- Remove `deadcode` and `depguard` linters
- Remove `structcheck` linter
- Remove `varcheck` linter

Signed-off-by: appleboy <appleboy.tw@gmail.com>

.golangci.yml

@@ -4,8 +4,6 @@ linters:
   fast: false
   enable:
     - bodyclose
-    - deadcode
-    - depguard
     - dogsled
     - dupl
     - errcheck
@@ -29,13 +27,11 @@ linters:
     - nolintlint
     - rowserrcheck
     - staticcheck
-    - structcheck
     - stylecheck
     - typecheck
     - unconvert
     - unparam
     - unused
-    - varcheck
     - whitespace
     - gofumpt