f2fs: cleanup not used anymore max_file_size()

Remove declaration of function max_file_size(). It was added
with:

63ba8a3d BACKPORT: f2fs: add a max block check for get_data_block_bmap

On merging of android-4.4@d68ba9f (4.4.89) functional part of
the above change was replaced with the upstream's one, which do
not use max_file_size() and also this function was renamed to
max_file_blocks() in its definition with change c1286ff4. In
android-4.4 this func was static, hence no declaration line in
f2fs.h and this is why it was left alone in msm-4.4 without to
be noticed.

c1286ff4 f2fs: backport from (4c1fad64 - Merge tag 'for-f2fs-4.9'
        of git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs

)

Change-Id: I197e8fa912f645f49acd1f97180944e303f3d314
Signed-off-by: Blagovest Kolenichev <bkolenichev@codeaurora.org>