Revert "c++: restore instantiate_decl assert"

In the testcase the assert fails because we use one member function from
another while we're in the middle of instantiating them all, which is
perfectly fine.  It seems complicated to detect this situation, so let's
remove the assert again.

	PR c++/109658

This reverts commit 95d4c0d2.

gcc/testsuite/ChangeLog:

	* g++.dg/template/local10.C: New test.