refactor(dpmodel): remove unused get_numb_attn_layer API

The compile guard that used this method was removed in 6d39ddf.

Author: Han Wang

deepmd/dpmodel/descriptor/dpa1.py

@@ -381,10 +381,6 @@ def get_dim_out(self) -> int:
     def get_dim_emb(self) -> int:
         return self.se_atten.dim_emb
 
-    def get_numb_attn_layer(self) -> int:
-        """Returns the number of se_atten attention layers."""
-        return self.se_atten.attn_layer
-
     def mixed_types(self) -> bool:
         """If true, the descriptor
         1. assumes total number of atoms aligned across frames;