[gdbserver, aarch64] Set usrregs to NULL in regs_info

[gdbserver, aarch64] Set usrregs to NULL in regs_info

Hi,
I happen to read the comments in regs_info below,

struct regs_info
{
  ...
  /* Info used when accessing registers with PTRACE_PEEKUSER /
     PTRACE_POKEUSER.  This can be NULL if all registers are
     transferred with regsets  .*/
  struct usrregs_info *usrregs;

that usrregs can be NULL if all registers are transferred with
regsets, which is exactly what aarch64-linux does.  This patch
is to set usrregs to NULL in regs_info and remove
aarch64_usrregs_info and aarch64_regmap.

gdb/gdbserver:

2015-07-09  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_regmap): Remove.
	(aarch64_usrregs_info): Remove.
	(regs_info): Set field usrregs to NULL.