[Ada] Cleanups related to front-end SJLJ

[Ada] Cleanups related to front-end SJLJ

This patch cleans up some code that is left over from the front-end SJLJ
exception handling mechanism, which has been removed.
This is in preparation for fixing a finalization-related bug.

Most importantly:

    The documentation is changed: a Handled_Sequence_Of_Statements node
    CAN contain both Exception_Handlers and an At_End_Proc.

    The assertion contradicting that is removed from
    Expand_At_End_Handler.

    The From_At_End field is removed.

gcc/ada/

	* sinfo.ads: Remove From_At_End.  Update comments.
	* gen_il-fields.ads, gen_il-gen-gen_nodes.adb, sem_ch11.adb:
	Remove From_At_End.
	* exp_ch11.adb (Expand_At_End_Handler): Remove assertion.
	* fe.h (Exception_Mechanism, Exception_Mechanism_Type, Has_DIC,
	Has_Invariants, Is_List_Member, List_Containing): Remove
	declarations that are not used in gigi.
	* opt.ads (Exception_Mechanism): This is not used in gigi.
	* exp_util.ads: Minor comment fix.