MM-31064: Simplify indentation (#16565)

* MM-31064: Simplify indentation

Reduce indentation where possible.

```release-note
NONE
```

Command ran to verify:
golangci-lint run --disable-all --enable golint --max-issues-per-linter=10000 --max-same-issues=100000 ./... | grep "block ends with a return state"

https://mattermost.atlassian.net/browse/MM-31064

* incorporate review comments

* enable golint for outdent rule

* fix remaining issues

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/utils.go

@@ -113,9 +113,8 @@ func (er *AppError) Translate(T goi18n.TranslateFunc) {
 func (er *AppError) SystemMessage(T goi18n.TranslateFunc) string {
 	if er.params == nil {
 		return T(er.Id)
-	} else {
-		return T(er.Id, er.params)
 	}
+	return T(er.Id, er.params)
 }
 
 func (er *AppError) ToJson() string {
@@ -136,11 +135,10 @@ func AppErrorFromJson(data io.Reader) *AppError {
 	decoder := json.NewDecoder(strings.NewReader(str))
 	var er AppError
 	err := decoder.Decode(&er)
-	if err == nil {
-		return &er
-	} else {
+	if err != nil {
 		return NewAppError("AppErrorFromJson", "model.utils.decode_json.app_error", nil, "body: "+str, http.StatusInternalServerError)
 	}
+	return &er
 }
 
 func NewAppError(where string, id string, params map[string]interface{}, details string, status int) *AppError {
@@ -258,9 +256,8 @@ func MapFromJson(data io.Reader) map[string]string {
 	var objmap map[string]string
 	if err := decoder.Decode(&objmap); err != nil {
 		return make(map[string]string)
-	} else {
-		return objmap
 	}
+	return objmap
 }
 
 // MapFromJson will decode the key/value pair map
@@ -270,9 +267,8 @@ func MapBoolFromJson(data io.Reader) map[string]bool {
 	var objmap map[string]bool
 	if err := decoder.Decode(&objmap); err != nil {
 		return make(map[string]bool)
-	} else {
-		return objmap
 	}
+	return objmap
 }
 
 func ArrayToJson(objmap []string) string {
@@ -286,9 +282,8 @@ func ArrayFromJson(data io.Reader) []string {
 	var objmap []string
 	if err := decoder.Decode(&objmap); err != nil {
 		return make([]string, 0)
-	} else {
-		return objmap
 	}
+	return objmap
 }
 
 func ArrayFromInterface(data interface{}) []string {
@@ -319,9 +314,8 @@ func StringInterfaceFromJson(data io.Reader) map[string]interface{} {
 	var objmap map[string]interface{}
 	if err := decoder.Decode(&objmap); err != nil {
 		return make(map[string]interface{})
-	} else {
-		return objmap
 	}
+	return objmap
 }
 
 func StringToJson(s string) string {
@@ -335,9 +329,8 @@ func StringFromJson(data io.Reader) string {
 	var s string
 	if err := decoder.Decode(&s); err != nil {
 		return ""
-	} else {
-		return s
 	}
+	return s
 }
 
 func GetServerIpAddress(iface string) string {
@@ -504,9 +497,8 @@ func GetImageMimeType(ext string) string {
 	ext = strings.ToLower(ext)
 	if len(IMAGE_MIME_TYPES[ext]) == 0 {
 		return "image"
-	} else {
-		return IMAGE_MIME_TYPES[ext]
 	}
+	return IMAGE_MIME_TYPES[ext]
 }
 
 func ClearMentionTags(post string) string {