testsuite: remove all Wstrict-overflow tests

We're no longer issuing Wstrict-overflow warnings, even with the
-Wstrict-overflow flag being used.

Remove the tests that are still testing for it.  They're either compile
tests that are testing cases where a warning shouldn't be issued or
XFAIL tests.

gcc/testsuite/ChangeLog:

	* gcc.dg/Wstrict-overflow-10.c: Removed.
	* gcc.dg/Wstrict-overflow-11.c: Removed.
	* gcc.dg/Wstrict-overflow-12.c: Removed.
	* gcc.dg/Wstrict-overflow-13.c: Removed.
	* gcc.dg/Wstrict-overflow-14.c: Removed.
	* gcc.dg/Wstrict-overflow-15.c: Removed.
	* gcc.dg/Wstrict-overflow-16.c: Removed.
	* gcc.dg/Wstrict-overflow-17.c: Removed.
	* gcc.dg/Wstrict-overflow-18.c: Removed.
	* gcc.dg/Wstrict-overflow-19.c: Removed.
	* gcc.dg/Wstrict-overflow-2.c: Removed.
	* gcc.dg/Wstrict-overflow-20.c: Removed.
	* gcc.dg/Wstrict-overflow-21.c: Removed.
	* gcc.dg/Wstrict-overflow-23.c: Removed.
	* gcc.dg/Wstrict-overflow-26.c: Removed.
	* gcc.dg/Wstrict-overflow-27.c: Removed.
	* gcc.dg/Wstrict-overflow-4.c: Removed.
	* gcc.dg/Wstrict-overflow-6.c: Removed.
	* gcc.dg/Wstrict-overflow-7.c: Removed.
	* gcc.dg/Wstrict-overflow-8.c: Removed.