fix: remove self_import from frontend (#1967)

Author: nicecui

docs/user-guide/deployments-administration/configuration.md

@@ -500,7 +500,7 @@ write_interval = "30s"
 
 #### `self_import` method
 
-Only `frontend` and `standalone` support exporting metrics using `self_import` method.
+If you are using `standalone`, you can use the `self_import` method to export metrics to GreptimeDB itself.
 
 ```toml
 [export_metrics]

i18n/zh/docusaurus-plugin-content-docs/current/user-guide/deployments-administration/configuration.md

@@ -490,7 +490,7 @@ write_interval = "30s"
 
 #### `self_import` 方法
 
-仅 `frontend` 和 `standalone` 支持使用 `self_import` 方法导出指标。
+如果你使用的是 GreptimeDB 单机版，可以使用 `self_import` 方法将指标导入到自身的数据库中。
 
 ```toml
 [export_metrics]

i18n/zh/docusaurus-plugin-content-docs/version-0.15/user-guide/deployments-administration/configuration.md

@@ -490,7 +490,7 @@ write_interval = "30s"
 
 #### `self_import` 方法
 
-仅 `frontend` 和 `standalone` 支持使用 `self_import` 方法导出指标。
+如果你使用的是 GreptimeDB 单机版，可以使用 `self_import` 方法将指标导入到自身的数据库中。
 
 ```toml
 [export_metrics]

versioned_docs/version-0.15/user-guide/deployments-administration/configuration.md

@@ -500,7 +500,7 @@ write_interval = "30s"
 
 #### `self_import` method
 
-Only `frontend` and `standalone` support exporting metrics using `self_import` method.
+If you are using `standalone`, you can use the `self_import` method to export metrics to GreptimeDB itself.
 
 ```toml
 [export_metrics]