kernel/iomem.c: remove __weak ioremap_cache helper

No portable code calls into this function any more, and on architectures
that don't use or define their own, it causes a warning:

kernel/iomem.c:10:22: warning: no previous prototype for 'ioremap_cache' [-Wmissing-prototypes]
   10 | __weak void __iomem *ioremap_cache(resource_size_t offset, unsigned long size)

Fold it into the only caller that uses it on architectures
without the #define.

Note that the fallback to ioremap is probably still wrong on
those architectures, but this is what it's always done there.

Link: https://lkml.kernel.org/r/20230726145432.1617809-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/iomem.c

@@ -5,18 +5,14 @@
 #include <linux/mm.h>
 #include <linux/ioremap.h>
 
-#ifndef ioremap_cache
-/* temporary while we convert existing ioremap_cache users to memremap */
-__weak void __iomem *ioremap_cache(resource_size_t offset, unsigned long size)
-{
-	return ioremap(offset, size);
-}
-#endif
-
 #ifndef arch_memremap_wb
 static void *arch_memremap_wb(resource_size_t offset, unsigned long size)
 {
+#ifdef ioremap_cache
 	return (__force void *)ioremap_cache(offset, size);
+#else
+	return (__force void *)ioremap(offset, size);
+#endif
 }
 #endif