gdb:csky rm tdesc_has_registers in csky_register_name

gdb:csky rm tdesc_has_registers in csky_register_name

As CSKY arch has not parsed target-description.xml in csky_gdbarch_init,
when a remote server, like csky-qemu or gdbserver, send a target-description.xml
to gdb, tdesc_has_registers will return ture, but tdesc_register_name (gdbarch, 0)
will return NULL, so a cmd "info registers r0" will not work.

Function of parsing target-description.xml will be add later for CSKY arch,
now it is temporarily removed to allow me to do other supported tests.

2021-07-15 Jiangshuai Li  <jiangshuai_li@c-sky.com>

            * csky-tdep.c : not using tdesc funtions in csky_register_name