Simplify regcache_cpy and remove regcache::cpy_no_passthrough

Simplify regcache_cpy and remove regcache::cpy_no_passthrough

Nowadays, regcache_cpy is used where src is read-only and dst is not
read-only, so the regcache_cpy can be simplified to handle this case only.
As a result, regcache::cpy_no_passthrough, which is about two read-only
regcache copy, is no longer used, remove it as well.

gdb:

2017-07-18  Yao Qi  <yao.qi@linaro.org>

	* regcache.c (regcache_cpy): Simplify it.
	(regcache::cpy_no_passthrough): Remove it.
	* regcache.h (cpy_no_passthrough): Remove it.
	(regcache_dup, regcache_cpy): Update comments.