Remove unused JavaThread functions for set_last_Java_fp/pc

- intptr_t* last_Java_fp() { return _anchor.last_Java_fp(); }
- void set_last_Java_fp(intptr_t* fp) { _anchor.set_last_Java_fp(fp); }
- void set_last_Java_pc(address pc) { _anchor.set_last_Java_pc(pc); }
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These functions are also unused and in the thread_platform.hpp files.