[Debugger] Removed DebuggerSession.set_Breakpoints to reduce complexity

[jstedfast]

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1026591/

Depends On:

• md-addins: https://github.com/xamarin/md-addins/pull/6245
• debugger-libs: https://github.com/mono/debugger-libs/pull/285

[jstedfast]

@monojenkins rebuild

[jstedfast]

Closing this since I'm not sure it's really needed. What we have no seems to be working.