ext4: remove unused function __ext4_write_dirty_metadata

The __ext4_write_dirty_metadata() function was introduced by commit
0390131b, "ext4: Allow ext4 to run without a journal", but nothing
ever used the function, either then or since.  So let's remove it and
save a bit of space.

Cc: Frank Mayhar <fmayhar@google.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>