Don't configure proxy from env twice

The proxy env vars are already taken into consideration here https://github.com/terraform-providers/terraform-provider-mysql/blob/master/mysql/provider.go#L152

Setting them as default for `proxy` leads to the proxy being set as proxy itself and as forwarder (see https://github.com/terraform-providers/terraform-provider-mysql/blob/master/mysql/provider.go#L160).

This leads to the following error (with `all_proxy=socks5://127.0.0.1:1080`)
Could not connect to server: socks connect tcp localhost:1080->target_host:target_port: socks connect tcp localhost: 1080->localhost: 1080

mysql/provider.go

@@ -63,10 +63,7 @@ func Provider() terraform.ResourceProvider {
 			"proxy": {
 				Type:     schema.TypeString,
 				Optional: true,
-				DefaultFunc: schema.MultiEnvDefaultFunc([]string{
-					"ALL_PROXY",
-					"all_proxy",
-				}, nil),
+				Default: nil,
 				ValidateFunc: validation.StringMatch(regexp.MustCompile("^socks5h?://.*:\\d+$"), "The proxy URL is not a valid socks url."),
 			},