[Darwin, testsuite] Remove unused test.

[Darwin, testsuite] Remove unused test.

During GCC-9, the codegen for unreachable switch case statements changed
such that the (undefined) behaviour of reaching such statements is directed
to one of the existing switch cases.  This means that the testcase which
deals with the old behaviour can no longer work (and there is nothing to test
with it).  The [Darwin-specific] test is now redundant and can be removed.

gcc/testsuite/

2019-07-12  Iain Sandoe  <iain@sandoe.co.uk>

	* gcc.dg/pr57438-2.c: Remove.

From-SVN: r273457