Don't use readUnspentCredits util in other primitive.

This clarifies that the only uses of readUnspentCredits are external to
the module. It's previous use in takeAllUnspentCredits was as part of a
CAS loop, which is really a primitive itself, and so it's misleading to
use the othe primitive here.

src/Database/LSMTree/Internal/MergingRun.hs

@@ -399,9 +399,8 @@ takeAllUnspentCredits ::
      PrimMonad m
   => UnspentCreditsVar (PrimState m)
   -> m Credits
-takeAllUnspentCredits
-    unspentCreditsVar@(UnspentCreditsVar !var) = do
-    prev <- readUnspentCredits unspentCreditsVar
+takeAllUnspentCredits (UnspentCreditsVar !var) = do
+    prev <- readPrimVar var
     casLoop prev
   where
     casLoop !prev = do