Remove unnecessary casts from TUI

Remove unnecessary casts from TUI

This removes a number of unnecessary casts from the TUI.  Some were
found with -Wuseless-cast (which, I think, can't easily be enabled for
gdb); and some were found by inspection.

Tested by rebuilding on x86-64 Fedora 28.
I'm checking this in.

gdb/ChangeLog
2018-10-15  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (strcat_to_buf): Remove casts.
	* tui/tui-winsource.c (tui_show_source_line)
	(tui_set_is_exec_point_at, tui_line_is_displayed): Remove casts.
	* tui/tui-wingeneral.c (tui_refresh_win, box_win): Remove casts.
	* tui/tui-windata.c (tui_first_data_item_displayed)
	(tui_delete_data_content_windows, tui_erase_data_content)
	(tui_display_all_data, tui_display_data_from)
	(tui_refresh_data_win, tui_vertical_data_scroll): Remove casts.
	* tui/tui-win.c (tui_set_win_height)
	(make_invisible_and_set_new_height, parse_scrolling_args): Remove
	casts.
	* tui/tui-win.c (tui_resize_all): Remove casts.
	(tui_scroll_backward_command, tui_set_focus)
	(tui_set_tab_width_command): Likewise.
	* tui/tui-source.c (tui_vertical_source_scroll): Remove cast.
	* tui/tui-regs.c (tui_show_register_group): Remove cast.
	* tui/tui-layout.c (tui_set_layout_by_name): Remove cast.
	* tui/tui-disasm.c (tui_vertical_disassem_scroll): Remove cast.
	* tui/tui-data.c (tui_partial_win_by_name, tui_free_win_content):
	Remove casts.