[Ada] Delete no-longer-used Convert_To_Return_False flag

[Ada] Delete no-longer-used Convert_To_Return_False flag

As a result of other recent changes, the Convert_To_Return_False flag
is never set. The flag can be therefore be deleted.

gcc/ada/

	* exp_ch11.adb (Expand_N_Raise_Expression): Remove
	Convert_To_Return_False test.
	* gen_il-fields.ads: Remove Convert_To_Return_False field.
	* gen_il-gen-gen_nodes.adb: Remove use of
	Convert_To_Return_False field.
	* sinfo.ads: Remove comment describing Convert_To_Return_False
	flag.