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

@@ -15,7 +15,7 @@
  * it guarantee that ALL functionality provided is working correctly.
  */
 
-#if defined(CONFIG_NEWLIB_LIBC) || defined(CONFIG_NATIVE_LIBC)
+#if defined(CONFIG_NATIVE_LIBC)
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L
 #endif