Remove stray sigma_delta.h header (#4523)
Remove stray sigma_delta.h header (#4523)

Remove stray, older version of sigma_delta.h

An older version of sigma_delta.h was committed to the root directory. Remove it, as we only want the later, new one already present in cores/esp8266. Thanks, @reaper7, for noticing it.
hallard/Arduino,sticilface/Arduino,hallard/Arduino,hallard/Arduino,lrmoreno007/Arduino,sticilface/Arduino,wemos/Arduino,sticilface/Arduino,Adam5Wu/Arduino,esp8266/Arduino,sticilface/Arduino,wemos/Arduino,hallard/Arduino,Adam5Wu/Arduino,Lan-Hekary/Arduino,Lan-Hekary/Arduino,Lan-Hekary/Arduino,lrmoreno007/Arduino,wemos/Arduino,hallard/Arduino,Adam5Wu/Arduino,esp8266/Arduino,esp8266/Arduino,Lan-Hekary/Arduino,Adam5Wu/Arduino,lrmoreno007/Arduino,lrmoreno007/Arduino,lrmoreno007/Arduino,esp8266/Arduino,wemos/Arduino,sticilface/Arduino,Adam5Wu/Arduino,Lan-Hekary/Arduino,esp8266/Arduino,wemos/Arduino