Remove split_style::UNDERSCORE

The recent changes to the way Ada names are matched means that
split_style::UNDERSCORE is no longer used.  This patch removes it.

gdb/split-name.c

@@ -45,19 +45,6 @@ split_name (const char *name, split_style style)
 	}
       break;
 
-    case split_style::UNDERSCORE:
-      /* Handle the Ada encoded (aka mangled) form here.  */
-      for (const char *iter = strstr (name, "__");
-	   iter != nullptr;
-	   iter = strstr (iter, "__"))
-	{
-	  result.emplace_back (&name[previous_len],
-			       iter - &name[previous_len]);
-	  iter += 2;
-	  previous_len = iter - name;
-	}
-      break;
-
     case split_style::DOT_STYLE:
       /* D and Go-style names.  */
       for (const char *iter = strchr (name, '.');

gdb/split-name.h

@@ -33,8 +33,6 @@ enum class split_style
   /* Split at ".".  Used by Ada, Go, D.  This has a funny name to work
      around a bug in Bison 2.3, which is used on macOS.  */
   DOT_STYLE,
-  /* Split at "__".  Used by Ada encoded names.  */
-  UNDERSCORE,
 };
 
 /* Split NAME into components at module boundaries.  STYLE indicates