bees: drop bees_sync, we will not need it

bees_sync() was an exception-trapping wrapper around fsync() which is
not needed in any of the contexts from which it was called:

	1.  dedupe operations implicitly flush the src data, so there is
	no need to call fsync() to do that twice.

	2.  crawl position is written to a temporary file and renamed
	over the original, which always forces a flush when the original
	exists.  On the first write, where there is no original, a
	crash would result in starting over with an empty or hole-filled
	beescrawl file, which is the initial state of bees.  There is also
	a long history of kernel bugs triggered by fsync() in this case.

	3.  we use unreadahead to trigger writeback for flushing the
	hash table to persistent storage.  Here is a space where we might
	use fsync after all, as part of bees_unreadahead's emulation of
	POSIX_FADV_DONTNEED, but we need to get read-once behavior from
	the scanner before we can use this capability.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -882,7 +882,6 @@ extern const char *BEES_USAGE;
 extern const char *BEES_VERSION;
 extern thread_local default_random_engine bees_generator;
 string pretty(double d);
-void bees_sync(int fd);
 void bees_readahead(int fd, off_t offset, size_t size);
 void bees_unreadahead(int fd, off_t offset, size_t size);
 string format_time(time_t t);