chore(linter): remove deprecated linters (matrix-org#3046)

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [x] I have justified why this PR doesn't need tests - linter setup
changes aren't something testable
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `Marcelina Hołub <[email protected]>`

Signed-off-by: Marcelina Hołub <[email protected]>
Co-authored-by: kegsay <[email protected]>

.golangci.yml

@@ -179,7 +179,6 @@ linters-settings:
 
 linters:
   enable:
-    - deadcode
     - errcheck
     - goconst
     - gocyclo
@@ -191,10 +190,8 @@ linters:
     - misspell # Check code comments, whereas misspell in CI checks *.md files
     - nakedret
     - staticcheck
-    - structcheck
     - unparam
     - unused
-    - varcheck
   enable-all: false
   disable:
     - bodyclose