Consistent syntax for class members
Currently, search.cpp uses three different syntax when accessing class member variables
* thisThread object references. This was a remnant from when pos.this_thread() was necessary for thread-local variable access
* this object references
* implicit member variable access
This PR aims to deprecate thisThread and standardize this syntax to implicit variable access, which is consistent with the rest of SF's codebase.
closes https://github.com/official-stockfish/Stockfish/pull/6154
no functional change