[lldb][nfc] Remove unused member Disassembler::m_base_addr (#89289)

This member variable is completely unused. I also don't think it makes a
ton of sense since (1) The "base address" can be obtained from the first
Instruction in its InstructionList, and (2) InstructionLists may not be
a series of contiguous instructions (even though they are most of the
time).