remove pre-commit hook

This was originally added so that unannex could prevent the hook from
running while files were in a state that the hook would interpret as
old-style unlocked and so would lock.

Now that's gone, so the only thing the hook was preventing was two
pre-commit processes running simulantaneously. But such concurrency
is normal in git-annex and should not be a problem.

Does mean that .git/hooks/pre-commit-annex might run more concurrently,
that seems the only risk of it causing any problems.

Command/PreCommit.hs

@@ -32,7 +32,7 @@ cmd = command "pre-commit" SectionPlumbing
 	(withParams seek)
 
 seek :: CmdParams -> CommandSeek
-seek ps = lockPreCommitHook $ do
+seek ps = do
 	l <- workTreeItems ps
 	-- fix symlinks to files being committed
 	flip withFilesToBeCommitted l $ \f -> commandAction $
@@ -74,7 +74,3 @@ showMetaDataChange = showLongNote . unlines . concatMap showmeta . fromMetaData
 	showset v
 		| isSet v = "+"
 		| otherwise = "-"
-
-{- Takes exclusive lock; blocks until available. -}
-lockPreCommitHook :: Annex a -> Annex a
-lockPreCommitHook = withExclusiveLock gitAnnexPreCommitLock