[lldb] Delete two overloads of Disassembler::Disassemble

by "inlining" them into their single caller (CommandObjectDisassemble).
The functions mainly consist of long argument lists and defensive
checks. These become unnecessary after inlining, so the end result is
less code. Additionally, this makes the implementation of
CommandObjectDisassemble more uniform (first figure out what you're
going to disassemble, then actually do it), which enables further
cleanups.