refactor: remove get_variable_bool and get_variable_number stubs.

* lib/zmacs/eval.lua (get_variable_bool, get_variable_number):
Remove.  Adjust all callers to use `get_variable` directly.
* lib/zz/eval.lua (get_variable_bool, get_variable_number):
Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>