parser: move atoi_n to import-divinglog.cpp

That was the only user of this helper function, so move it there.
Moreover, impelement it with the standard function std::from_chars
instead of copying the string.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/parse.cpp

@@ -466,15 +466,3 @@ void add_dive_site(const char *ds_name, struct dive *dive, struct parser_state *
 		}
 	}
 }
-
-int atoi_n(char *ptr, unsigned int len)
-{
-	if (len < 10) {
-		char buf[10];
-
-		memcpy(buf, ptr, len);
-		buf[len] = 0;
-		return atoi(buf);
-	}
-	return 0;
-}