[Ada] Remove redundant guard against Any_String

[Ada] Remove redundant guard against Any_String

A sequence of checks for a valid Boolean argument fails when applied to
Any_String because its component is not of a Boolean type. The explicit
guard was unnecessary; it was only needed when a First_Index applied to
Any_String would crash, but this was fixed soon after this guard was
added.

Cleanup related to recent fixes for resolution of Boolean operators.

gcc/ada/

	* sem_type.adb (Valid_Boolean_Arg): Remove redundant guard.