c++: Remove obsolete dg-module-headers [PR 99023]

c++: Remove obsolete dg-module-headers [PR 99023]

PR99023's testcase is highlighting some missing functionality of the
modules test harness.  I did have some partial support, but it's only
use in one place for a now-obsolete test.  This patch expunges that
support so I can add better functionality now I understand better what
is necessary.

	PR c++/99023
	gcc/testsuite/
	* g++.dg/modules/modules.exp: Remove dg-module-headers support
	* g++.dg/modules/alias-2_a.H: Delete.
	* g++.dg/modules/sys/alias-2_a.H: Delete.