Clean up PGTK code more

Clean up PGTK code more

* src/pgtkterm.c (STORE_KEYSYM_FOR_DEBUG): Delete macro.
(x_find_modifier_meanings): Delete function.
(get_modifier_values, pgtk_gtk_to_emacs_modifiers)
(pgtk_emacs_to_gtk_modifiers): Use GDK's own modifier mask
constants instead of detecting our own from the keymap.  This
does mean a specific "meta" key will no longer be detected on X
Windows, but that interface doesn't exist on Wayland, and X
users should use the X port anyway.
(key_press_event): Avoid decoding input text.
(pgtk_term_init): Stop computing modifiers.
(syms_of_pgtkterm): Delete obsolete comment.
* src/pgtkterm.h (struct pgtk_display_info): Delete modifier
masks.