Ada: Fix instantiation failure with formal array type after formal package

This comes from a confusion in the mapping maintained between formal and
actual parameters of the instantiation caused by the equivalent mapping
maintained for the instantiation of the formal package.  The change just
removes the offending lines, which do not seem to serve any useful purpose.

gcc/ada/
	PR ada/124606
	* sem_ch12.adb (Find_Actual_Type): Rename formal parameter.
	(Map_Formal_Package_Entities): Do not register base types.

gcc/testsuite/
	* gnat.dg/generic_inst19.adb: New test.