tests/c_lib: Remove define of _POSIX_C_SOURCE for newlib

This test does not use any APIs beyond those provided in the Zephyr
standard set, so it should not define _POSIX_C_SOURCE.

Signed-off-by: Keith Packard <keithp@keithp.com>

tests/lib/c_lib/common/src/main.c

@@ -15,10 +15,6 @@
  * it guarantee that ALL functionality provided is working correctly.
  */
 
-#ifdef CONFIG_NEWLIB_LIBC
-#define _POSIX_C_SOURCE 200809L
-#endif
-
 #include <zephyr/kernel.h>
 #include <zephyr/sys/__assert.h>
 #include <zephyr/ztest.h>