systemc: Make verify.py delete obsolete diff files.
systemc: Make verify.py delete obsolete diff files.

If diff file exists but the underlying diff has been fixed, delete the
diff file.

Change-Id: Icadc21a61c084198a8a246ab6d00a9b885647cde
Reviewed-on: https://gem5-review.googlesource.com/12056
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu