Remove all abi dumps, not just ones shared between versions

While the abi-checking script handled comparing only the modules
that were shared between the old and new versions correctly, the
cleanup of the abi dumps only removed what was shared. Change the
cleanup logic to remove all abi dumps instead.
diff --git a/scripts/abi_check.py b/scripts/abi_check.py
index f837f7a..2b3c1bd 100755
--- a/scripts/abi_check.py
+++ b/scripts/abi_check.py
@@ -279,8 +279,9 @@
                 )
                 if not (self.keep_all_reports or self.brief):
                     os.remove(output_path)
-            os.remove(self.old_version.abi_dumps[mbed_module])
-            os.remove(self.new_version.abi_dumps[mbed_module])
+        for version in [self.old_version, self.new_version]:
+            for mbed_module, mbed_module_dump in version.abi_dumps.items():
+                os.remove(mbed_module_dump)
         if self.can_remove_report_dir:
             os.rmdir(self.report_dir)
         self.log.info(compatibility_report)