Remove sh5media divtab code

Spurred by Akari Takahashi's patch to config/sh/divtab.cc, this removes
divtab.cc completely.

divtab.cc was used to calculate a division table for the sh5 media
processor.  GCC dropped support for that (unmanufactured) chip back
in 2016 and this file simply got missed AFAICT.

gcc/
	* config/sh/divtab.cc: Remove.