[Ada] Remove kludge for validity checks on Long_Float type

[Ada] Remove kludge for validity checks on Long_Float type

This patch reverts a fix for a spurious warning for validity checks on
type Long_Float. This fix was dubious (as it was only affecting
Long_Float and not Float) and apparently is no longer needed.

Cleanup related to improved detection of uninitialised scalar objects.

gcc/ada/

	* sem_attr.adb (Note_Possible_Modification): Revert a
	special-case for validity checks on Long_Float type.
	* snames.ads-tmpl (Name_Attr_Long_Float): Remove name added
	exclusively for the mentioned fix.