Remove IsLIBGAP constant

The presence of the `--noshell` command line option makes the `IsLIBGAP`
constant obsolete, so we remove it.

lib/init.g

@@ -938,16 +938,6 @@ BindGlobal ("ShowSystemInformation", function ()
 end );
 
 
-#############################################################################
-##
-## Initialize the IsLIBGAP variable (if not done before). If this variable
-## is false, an interactive session will be started.
-## Otherwise no interactive session is started.
-##
-if not IsBound( IsLIBGAP ) then
-  BIND_CONSTANT( "IsLIBGAP", false );
-fi;
-
 #############################################################################
 ##
 ##  Finally, deal with the lists of global variables.
@@ -1037,8 +1027,9 @@ InstallAndCallPostRestore( function()
     od;
 end );
 
-if IsLIBGAP then
-  # GAP is used as a library, do not start an interactive session
+
+if GAPInfo.CommandLineOptions.noshell then
+  # do not start an interactive session
 elif IsHPCGAP and THREAD_UI() then
   ReadLib("hpc/consoleui.g");
   MULTI_SESSION();