bees: clean up #if 0 ... fsync ... #endif code

Remove some dead code because dedup-related deadlocks have not been
observed since Linux kernel v4.11.

Preserve rationale of remaining #if 0 block (why we do write/rename
instead of write/fsync/rename) so that people don't try to replace the
"missing" fsync() there.

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

src/bees-context.cc

@@ -173,13 +173,6 @@ BeesContext::dedup(const BeesRangePair &brp)
 	brp.first.fd(shared_from_this());
 	brp.second.fd(shared_from_this());
 
-#if 0
-	// This avoids some sort of kernel race condition;
-	// however, it also doubles our dedup times.
-	// Is avoiding a crash every few weeks worth it?
-	bees_sync(brp.first.fd());
-#endif
-
 	BEESTOOLONG("dedup " << brp);
 
 	BeesAddress first_addr(brp.first.fd(), brp.first.begin());

src/bees.cc

@@ -416,6 +416,12 @@ BeesStringFile::write(string contents)
 		// This triggers too many btrfs bugs.  I wish I was kidding.
 		// Forget snapshots, balance, compression, and dedup:
 		// the system call you have to fear on btrfs is fsync().
+		// Also note that when bees renames a temporary over an
+		// existing file, it flushes the temporary, so we get
+		// the right behavior if we just do nothing here
+		// (except when the file is first created; however,
+		// in that case the result is the same as if the file
+		// did not exist, was empty, or was filled with garbage).
 		BEESNOTE("fsyncing " << tmpname << " in " << name_fd(m_dir_fd));
 		DIE_IF_NON_ZERO(fsync(ofd));
 #endif