[Ada] Remove useless code in Backend_Processing_Possible

[Pierre-Marie de Rodat]

The call to Set_Size_Known_At_Compile_Time in
Backend_Processing_Possible happened just after querying
Size_Known_At_Compile_Time several lines before. Both calls operated on
the same type entity, so the call to "Set" routine had no effect and now
is removed.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-11  Piotr Trojanek  <troja...@adacore.com>

gcc/ada/

        * exp_aggr.adb (Backend_Processing_Possible): Remove useless
        call.
--- gcc/ada/exp_aggr.adb
+++ gcc/ada/exp_aggr.adb
@@ -743,7 +743,6 @@ package body Exp_Aggr is
 
       --  Backend processing is possible
 
-      Set_Size_Known_At_Compile_Time (Etype (N), True);
       return True;
    end Backend_Processing_Possible;