gdb/tui: Remove casts of NULL during assignment.

gdb/tui: Remove casts of NULL during assignment.

In the following code:

    struct symbol *wsym = (struct symbol *) NULL;

the cast of NULL is redundant, it adds noise, and is just one more thing
to change if the type of wsym ever changes.  There are a relatively
small number of places in gdb where the above code pattern is used.
Usually the cast is removed like this:

    struct symbol *wsym = NULL;

This commit updates all the places within the gdb/tui directory where we
cast NULL during assignment, removing the cast.

gdb/ChangeLog:

	* tui/tui-data.c (win_with_focus): Remove cast of NULL pointer.
	(tui_next_win): Likewise.
	(tui_prev_win): Likewise.
	(tui_partial_win_by_name): Likewise.
	(tui_init_generic_part): Likewise.
	(init_content_element): Likewise.
	(tui_del_window): Likewise.
	(tui_free_window): Likewise.
	(tui_del_data_windows): Likewise.
	(tui_free_data_content): Likewise.
	* tui/tui-layout.c (make_source_or_disasm_window): Likewise.
	* tui/tui-regs.c (tui_show_register_group): Likewise.
	* tui/tui-win.c (tui_resize_all): Likewise.
	(tui_set_focus): Likewise.
	(tui_set_win_height): Likewise.
	(make_invisible_and_set_new_height): Likewise.
	* tui/tui-windata.c (tui_delete_data_content_windows): Likewise.
	* tui/tui-wingeneral.c (make_visible): Likewise.