gdb: cleanup around some set_momentary_breakpoint_at_pc calls

I noticed a couple of places in infrun.c where we call
set_momentary_breakpoint_at_pc, and then set the newly created
breakpoint's thread field, these are in:

  insert_exception_resume_breakpoint
  insert_exception_resume_from_probe

Function set_momentary_breakpoint_at_pc calls
set_momentary_breakpoint, which always creates the breakpoint as
thread-specific for the current inferior_thread().

The two insert_* functions mentioned above take an arbitrary
thread_info* as an argument and set the breakpoint::thread to hold the
thread number of that arbitrary thread.

However, the insert_* functions store the breakpoint pointer within
the current inferior_thread(), so we know that the thread being passed
in must be the currently selected thread.

What this means is that we can:

  1. Assert that the thread being passed in is the currently selected
  thread, and

  2. No longer adjust the breakpoint::thread field, this will already
  have been set correctly be calling set_momentary_breakpoint_at_pc.

There should be no user visible changes after this commit.