Remove duplicate millis/micros definitions (#419)

Fixes #418

cores/rp2040/millis.cpp

-/*
-    Millis() for the Raspberry Pi Pico RP2040
-
-    Copyright (c) 2021 Earle F. Philhower, III <earlephilhower@yahoo.com>
-
-    This library is free software; you can redistribute it and/or
-    modify it under the terms of the GNU Lesser General Public
-    License as published by the Free Software Foundation; either
-    version 2.1 of the License, or (at your option) any later version.
-
-    This library is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-    Lesser General Public License for more details.
-
-    You should have received a copy of the GNU Lesser General Public
-    License along with this library; if not, write to the Free Software
-    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
-*/
-
-#include <pico/time.h>
-
-extern "C" unsigned long millis() {
-    return to_ms_since_boot(get_absolute_time());
-}
-
-extern "C" unsigned long micros() {
-    return time_us_32();
-}