ada: Fix infinite loop on MSP430 with -mlarge flag

This removes the loop trying to find a pointer mode among the integer modes,
which is obsolete and does not work on platforms where pointers have unusual
size like MSP430 or special semantics like Morello.

gcc/ada/ChangeLog:
	PR ada/116498
	* gcc-interface/decl.cc (validate_size): Use the size of the default
	pointer mode as the minimum size for access types and fat pointers.