mm: remove READ_ONLY_THP_FOR_FS Kconfig option

After removing READ_ONLY_THP_FOR_FS check in file_thp_enabled(),
khugepaged and MADV_COLLAPSE can run on FSes with PMD THP pagecache
support even without READ_ONLY_THP_FOR_FS enabled.  Remove the Kconfig
first so that no one can use READ_ONLY_THP_FOR_FS as upcoming commits
remove mapping->nr_thps, which its safe guard mechanism relies on.

Link: https://lore.kernel.org/20260517135416.1434539-6-ziy@nvidia.com
Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Nico Pache <npache@redhat.com>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Sterba <dsterba@suse.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Liam Howlett <liam@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/Kconfig

@@ -936,17 +936,6 @@ config THP_SWAP
 
 	  For selection by architectures with reasonable THP sizes.
 
-config READ_ONLY_THP_FOR_FS
-	bool "Read-only THP for filesystems (EXPERIMENTAL)"
-	depends on TRANSPARENT_HUGEPAGE
-
-	help
-	  Allow khugepaged to put read-only file-backed pages in THP.
-
-	  This is marked experimental because it is a new feature. Write
-	  support of file THPs will be developed in the next few release
-	  cycles.
-
 config NO_PAGE_MAPCOUNT
 	bool "No per-page mapcount (EXPERIMENTAL)"
 	help