OAuth: Removes send_client_credentials_via_post setting (#20044)

Removes send_client_credentials_via_post oauth setting and 
use auto-detect mechanism instead.
By these changes also fixes statichcheck errors

Ref #8968

pkg/setting/setting_oauth.go

@@ -1,23 +1,22 @@
 package setting
 
 type OAuthInfo struct {
-	ClientId, ClientSecret       string
-	Scopes                       []string
-	AuthUrl, TokenUrl            string
-	Enabled                      bool
-	EmailAttributeName           string
-	EmailAttributePath           string
-	RoleAttributePath            string
-	AllowedDomains               []string
-	HostedDomain                 string
-	ApiUrl                       string
-	AllowSignup                  bool
-	Name                         string
-	TlsClientCert                string
-	TlsClientKey                 string
-	TlsClientCa                  string
-	TlsSkipVerify                bool
-	SendClientCredentialsViaPost bool
+	ClientId, ClientSecret string
+	Scopes                 []string
+	AuthUrl, TokenUrl      string
+	Enabled                bool
+	EmailAttributeName     string
+	EmailAttributePath     string
+	RoleAttributePath      string
+	AllowedDomains         []string
+	HostedDomain           string
+	ApiUrl                 string
+	AllowSignup            bool
+	Name                   string
+	TlsClientCert          string
+	TlsClientKey           string
+	TlsClientCa            string
+	TlsSkipVerify          bool
 }
 
 type OAuther struct {