common/thread: Remove unused functions

Many of these functions are carried over from Dolphin (where they aren't
used anymore). Given these have no use (and we really shouldn't be
screwing around with OS-specific thread scheduler handling from the
emulator, these can be removed.

The function for setting the thread name is left, however, since it can
have debugging utility usages.

src/common/thread.h

@@ -9,7 +9,6 @@
 #include <cstddef>
 #include <mutex>
 #include <thread>
-#include "common/common_types.h"
 
 namespace Common {
 
@@ -78,9 +77,6 @@ private:
     std::size_t generation = 0; // Incremented once each time the barrier is used
 };
 
-void SetThreadAffinity(std::thread::native_handle_type thread, u32 mask);
-void SetCurrentThreadAffinity(u32 mask);
-void SwitchCurrentThread(); // On Linux, this is equal to sleep 1ms
 void SetCurrentThreadName(const char* name);
 
 } // namespace Common