Revert "parisc: Export flush_cache_page() (needed by lustre)"

This reverts commit 320c90be.

Christoph Hellwig <hch@infradead.org> commented:
This one shouldn't go in - Geert sent it a bit prematurely, as Lustre
shouldn't use it just to reimplement core VM functionality (which it
shouldn't use either, but that's a separate story).

Signed-off-by: Helge Deller <deller@gmx.de>