c++/modules: Remove incorrect assertion [PR122015,PR122019]

This assertion, despite what I said in r16-4070, is not valid: we can
reach here when deduping a VAR_DECL that didn't get a LANG_SPECIFIC in
the current TU.  It's still correct to always use lang_cplusplus however
as for anything else the decl would have been created with an
appropriate LANG_SPECIFIC to start with.

	PR c++/122015
	PR c++/122019

gcc/cp/ChangeLog:

	* module.cc (trees_in::install_entity): Remove incorrect
	assertion.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
(cherry picked from commit 4f9f1269f4e929ee471b717b47c33d2d5a3db0d3)