coroutine: Remove problematic diagnostic from api-3 test

coroutine: Remove problematic diagnostic from api-3 test

The api-3 testcase devotes most of its available stack space to a test
buffer, leaving only a small amount (COROUTINE_MIN_STKSZ) for the actual
stack usage of the coroutine.

It turns out that the ccan/tap diag() function can - depending on compiler
version and flags, and on whether diagnostics are enabled - exceed that
limited stack space.  That leads to a stack overrun, and in turn corruption
of the parent routine's stack, generating unpredictable and hard to debug
SEGVs.

At present, this bug seems to be tripped by clang-3.8 when diagnostic
messages are printed.

This removes the troublesome diag() call.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>