cpu/lpc2387: rtc: remove _rtc_set()
cpu/lpc2387: rtc: remove _rtc_set()

Calling localtime() adds considerable overhead.
There are easier ways to set the date to 1970.

For tests/periph_rtc this results in this ROM change:

master:

   text    data     bss     dec     hex
  31328     240   98064  129632   1fa60

with this patch:

   text    data     bss     dec     hex
  20036     140   98168  118344   1ce48

ant9000/RIOT,kYc0o/RIOT,yogo1212/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,kYc0o/RIOT,basilfx/RIOT,miri64/RIOT,kYc0o/RIOT,smlng/RIOT,ant9000/RIOT,ant9000/RIOT,basilfx/RIOT,yogo1212/RIOT,smlng/RIOT,smlng/RIOT,miri64/RIOT,jasonatran/RIOT,OlegHahm/RIOT,yogo1212/RIOT,kaspar030/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,ant9000/RIOT,OlegHahm/RIOT,authmillenon/RIOT,basilfx/RIOT,OlegHahm/RIOT,basilfx/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,kaspar030/RIOT,basilfx/RIOT,smlng/RIOT,RIOT-OS/RIOT,smlng/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,yogo1212/RIOT,kaspar030/RIOT,OlegHahm/RIOT