libc: replace null.h and size_t.h with stddef.h

According to commit b71a8a45 and Jira: ZEP-733
(libc: remove stddef.h which is provided by the compiler),
the stddef.h in zephyr code is removed and it's provided
by the compiler. The original stddef.h includes two head
files, null.h and size_t.h, which are also useless now.
So remove these two deprecated files, in case conflicting
definition.

Change-Id: Ie7163fdbd23c32759425b50f3deff2a57cc051a9
Signed-off-by: Qiu Peiyang <peiyangx.qiu@intel.com>