This removes definitions relating to the built-in SNTP client that
are LwIP v1 specific. Instead of duplicating these pull in the
LwIP header that correspond to the required functions depending on
the version of the stack being used.
Without this fix calls to sntp_getserver() work but return invalid
data and can lead to stack exhaustion.
Update the NTP-TZ-DST example to use the Arduino sntp.h header
rather than duplicate the conditional checks to use the LwIP header.
Tests:
- Build against a simple SNTP API demonstratin app and all
LwIP configurations. Verify that the app runs for an extended
period and that the expected results are obtained.
