kunit: remove KUNIT_INIT_MEM_ASSERTION macro

Commit 870f63b7cd78 ("kunit: eliminate KUNIT_INIT_*_ASSERT_STRUCT
macros") removed all the other macros of this type.

But it raced with commit b8a926be ("kunit: Introduce
KUNIT_EXPECT_MEMEQ and KUNIT_EXPECT_MEMNEQ macros"), which added another
instance.

Remove KUNIT_INIT_MEM_ASSERTION and just use the generic
KUNIT_INIT_ASSERT macro instead.
Rename the `size` arg to avoid conflicts by appending a "_" (like we did
in the previous commit).

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>