[lldb/Utility] Remove m_ieee_quad from Scalar

[lldb/Utility] Remove m_ieee_quad from Scalar

This field is unused (the only way to change its value is via a
constructor which is never called), and as far as I can tell it has been
unused since it was introduced in D12100. It also has some soundness
issues -- e.g.  operator= does not reinitialize it, but uses the old
value from the overwritten object.

It sounds like this class should be able to support different floating
point semantics, but if that is needed, it would be better to start
afresh -- probably by passing in an APFloat::fltSemantics object instead
of a bool flag.