ada: Remove obsolete code in Analyze_Assignment

This code was dealing with build-in-place calls for nonlimited types, but
they no longer exist since Is_Build_In_Place_Result_Type => Is_Limited_View.

gcc/ada/

	* sem_ch5.adb (Analyze_Assignment): Turn Rhs into a constant and
	remove calls to the following subprograms.
	(Transform_BIP_Assignment): Delete.
	(Should_Transform_BIP_Assignment): Likewise.