scons: Remove an extraneous Exit().

This isn't necessary after error() which exits on its own.

Change-Id: Icad08c1badc73fa8f41013cc69d6cc5a96ff8fdb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40961
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>

SConstruct

@@ -353,7 +353,6 @@ if main['GCC']:
     if compareVersions(main['CXXVERSION'], "5") < 0:
         error('gcc version 5 or newer required.\n'
               'Installed version:', main['CXXVERSION'])
-        Exit(1)
 
     # Add the appropriate Link-Time Optimization (LTO) flags
     # unless LTO is explicitly turned off.