Remove dead code in regcache::dump

footnote_register_size in regcache::dump is a constant zero, so the
condition check against footnote_register_size is dead code.  The code
writing to footnote_register_size was removed by 01e1877c.

This patche removes footnote_register_size and the dead code.

gdb:

2017-11-24  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache::dump): Remove footnote_register_size.