Remove some TUI asserts

This removes a few asserts from the TUI.  These asserts aren't useful,
because they simply check an invariant that's already ensured by the
type system.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_source_window_base::set_is_exec_point_at)
	(tui_source_window_base::update_breakpoint_info): Remove asserts.

Change-Id: I807e1e9bdb0cfa475e70375ceca3a5d4f2eb8d0b