Remove page_was_dirty and remove_protection duplicate definitions

Remove page_was_dirty and remove_protection duplicate definitions

* include/private/gc_priv.h [!GC_DISABLE_INCREMENTAL]
(GC_page_was_dirty): Refine comment (move the comment
content from GC_page_was_dirty in os_dep.c).
* os_dep.c [GWW_VDB && MPROTECT_VDB] (GC_gww_page_was_dirty): Remove.
* os_dep.c [GWW_VDB || MPROTECT_VDB || PROC_VDB || MANUAL_VDB]
(GC_page_was_dirty): Avoid code duplication (remove the functions of
the identical implementation).
* os_dep.c [(GWW_VDB || PROC_VDB) && !MPROTECT_VDB || MANUAL_VDB
|| DEFAULT_VDB] (GC_remove_protection): Likewise.
* os_dep.c [CHECKSUMS && GWW_VDB || PROC_VDB] (GC_or_pages): Remove
nesting of #if.