debugger: Improve clearBreakpoint

This will make clearBreakpoint() to clear breakpoint on current line(no
arguments) and clearBreakpoint(number) to clear breakpoint on the line
number.

For #7026

Right now, I'm working on getting setBreakpoint('functionname()') working. I get this error:
```
debug> sb('someFn()')
Error: "ReferenceError: someFn is not defined" evaluating "someFn"
```