mboxd: Remove flash API compatibility shim

The flash API compatibility was kept to reduce the line noise in the
previous backend patch. Remove the compatibility layer now and convert
the remaining call-sites.

Change-Id: I4b6e54f4463059a7804918add81e7572db7b7c21
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/test/flash_copy.c b/test/flash_copy.c
index 472315b..9300cff 100644
--- a/test/flash_copy.c
+++ b/test/flash_copy.c
@@ -71,7 +71,7 @@
 
 	assert(!backend_probe_mtd(&context.backend, tmp.path));
 
-	flash_copy(&context, 0, dst, TEST_SIZE);
+	backend_copy(&context.backend, 0, dst, TEST_SIZE);
 	assert(0 == memcmp(src, dst, TEST_SIZE));
 
 	backend_free(&context.backend);