Remove Chainstate::LoadMempool

The 3-line function is only called once outside of tests, so it is
clearer to inline it.

src/validation.h

@@ -712,9 +712,6 @@ public:
     /** Find the last common block of this chain and a locator. */
     const CBlockIndex* FindForkInGlobalIndex(const CBlockLocator& locator) const EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
-    /** Load the persisted mempool from disk */
-    void LoadMempool(const fs::path& load_path, fsbridge::FopenFn mockable_fopen_function = fsbridge::fopen);
-
     /** Update the chain tip based on database information, i.e. CoinsTip()'s best block. */
     bool LoadChainTip() EXCLUSIVE_LOCKS_REQUIRED(cs_main);