clean useless comment, test=document_fix (#52364)

paddle/phi/common/backend.h

@@ -73,30 +73,6 @@ enum class Backend : uint8_t {
    * and some kernels are also device-independent when implemented based on
    * primitive API.
    *
-   * In this case, we need to provide a more concise registration method,
-   * instead of registering the kernels for each device with almost
-   * repetitive code, we need one registration covers all situations,
-   * so if we provide the ALL field with Register the kernel in this statement.
-   *
-   * Of course, we have also considered solving this problem through different
-   * named macros, for example, if we define
-   *
-   * PD_REGISTER_KERNEL_FOR_ALL_BACKEND
-   *
-   * Based on this design pattern, the dtype and layout also have the same
-   * requirements, this cause we need to define a series of macros
-   *
-   * PD_REGISTER_KERNEL_FOR_ALL_DTYPE
-   * PD_REGISTER_KERNEL_FOR_ALL_LAYOUT
-   * PD_REGISTER_KERNEL_FOR_ALL_BACKEND_AND_LAYOUT
-   * PD_REGISTER_KERNEL_FOR_ALL_BACKEND_AND_DTYPE
-   * PD_REGISTER_KERNEL_FOR_ALL_LAYOUT_AND_DTYPE
-   * PD_REGISTER_KERNEL_FOR_ALL_BACKEND_AND_LAYOUT_AND_DTYPE
-   *
-   * It makes the system of registering macros more complicated, we think
-   * this is not a simple design, so we still adopt the design of providing
-   * the ALL field.
-   *
    * Note: ALL_BACKEND only used for Kernel registration and selection
    */
   ALL_BACKEND = UNDEFINED,