thp: remove HPAGE_PMD_ORDER minimum assertion

We now handle order-1 folios correctly, so we don't need this assertion
any more.

Link: https://lkml.kernel.org/r/20240429190114.3126789-1-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>