[lldb] Fix Disasembler build error on 32-bit systems
After changes in https://github.com/llvm/llvm-project/pull/145793.
/home/david.spickett/llvm-project/lldb/source/Plugins/Disassembler/LLVMC/DisassemblerLLVMC.cpp:1360:49: error: non-const lvalue reference to type 'uint64_t' (aka 'unsigned long long') cannot bind to a value of unrelated type 'size_t' (aka 'unsigned int')
1360 | status = m_disasm_up->getInstruction(mc_inst, size, data, pc, llvm::nulls());
| ^~~~
/home/david.spickett/llvm-project/llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h:135:64: note: passing argument to parameter 'Size' here
135 | virtual DecodeStatus getInstruction(MCInst &Instr, uint64_t &Size,
| ^
1 error generated.
The type used in the LLVM method we call is uin64_t so use that instead.
It's overkill for what it is, but that's a separate issue if anyone cares.
Also removed the unused form of GetMCInst.
