Remove exception_none

Now that gdb_exception has a constructor, there's no need for
exception_none.  This patch removes it.

gdb/ChangeLog
2019-04-25  Tom Tromey  <tromey@adacore.com>

	* xml-support.c (gdb_xml_parser::gdb_xml_parser): Update.
	* python/py-value.c (valpy_getitem, valpy_nonzero): Update.
	* python/py-inferior.c (infpy_write_memory, infpy_search_memory):
	Update.
	* python/py-breakpoint.c (bppy_set_condition, bppy_set_commands):
	Update.
	* mi/mi-interp.c (mi_interp::exec): Update.
	* linespec.c (parse_linespec): Update.
	* infcall.c (run_inferior_call): Update.
	* guile/scm-value.c (gdbscm_value_to_lazy_string): Update.
	* guile/scm-symbol.c (gdbscm_lookup_symbol)
	(gdbscm_lookup_global_symbol): Update.
	* guile/scm-param.c (gdbscm_parameter_value): Update.
	* guile/scm-frame.c (gdbscm_frame_read_register)
	(gdbscm_frame_read_var): Update.
	* guile/scm-breakpoint.c (gdbscm_register_breakpoint_x): Update.
	* exec.c (try_open_exec_file): Update.
	* event-top.c (gdb_rl_callback_read_char_wrapper_noexcept)
	(gdb_rl_callback_handler): Update.
	* common/common-exceptions.h (exception_none): Don't declare.
	* common/common-exceptions.c (exception_none): Don't define.
	(struct catcher) <exception>: Update.
	* cli/cli-interp.c (safe_execute_command): Update.
	* breakpoint.c (insert_bp_location, location_to_sals): Update.