Cleanup for thread safety

Compiling with Clang 15 caught an extra unneeded lock
on cs_main and also an unnecessary exclusive lock required
on IsV13Enabled. The gArgs global methods are thread-safe via
the internal cs in ArgsManager.

src/chainparams.h

@@ -153,7 +153,7 @@ inline bool IsV12Enabled(int nHeight)
     return nHeight >= Params().GetConsensus().BlockV12Height;
 }
 
-inline bool IsV13Enabled(int nHeight) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+inline bool IsV13Enabled(int nHeight)
 {
     // The argument driven override temporarily here to facilitate testing.
 

src/qt/mrcmodel.cpp

@@ -255,8 +255,6 @@ void MRCModel::refresh() EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 
     // This is similar to createmrcrequest in many ways, but the state tracking is more complicated.
 
-    LOCK(cs_main);
-
     // Record initial block height during init run.
     if (!m_init_block_height) {
         m_init_block_height = pindexBest->nHeight;