Bignum Tests: remove ModOperationCommonArchSplit

The functionality of ModOperationCommonArchSplit is needed in several
subclasses, therefore moving it to a superclass.

There is another, redundant ArchSplit class, which will be removed in a
later commit.

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/scripts/mbedtls_dev/bignum_mod_raw.py b/scripts/mbedtls_dev/bignum_mod_raw.py
index 58a93fc..f44acef 100644
--- a/scripts/mbedtls_dev/bignum_mod_raw.py
+++ b/scripts/mbedtls_dev/bignum_mod_raw.py
@@ -50,12 +50,13 @@
 # END MERGE SLOT 6
 
 # BEGIN MERGE SLOT 7
-class BignumModRawConvertToMont(bignum_common.ModOperationCommonArchSplit,
+class BignumModRawConvertToMont(bignum_common.ModOperationCommon,
                                 BignumModRawTarget):
     """ Test cases for mpi_mod_raw_to_mont_rep(). """
 
     test_function = "mpi_mod_raw_to_mont_rep"
     test_name = "Convert into Mont: "
+    arch_split = True
 
     test_data_moduli = ["b",
                         "fd",