xtensa: userspace: remove saving HiFi registers for syscalls

This removes the call to _xtensa_hifi_save() to save the HiFi
registers during syscalls. During syscall, we are not doing
context switching, and technically it is still the same thread.
There is no need to save HiFi registers.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>