arch: Get rid of the generic mmapped IPR mechanism.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I4ab6f80581eee39e90fb91c672eca8e1a8fd9046
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23186
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/arch/generic/SConscript

@@ -44,7 +44,6 @@ if env['TARGET_ISA'] == 'null':
     Return()
 
 Source('decode_cache.cc')
-Source('mmapped_ipr.cc')
 
 SimObject('BaseInterrupts.py')
 SimObject('BaseISA.py')