Remove superfluous function key_is_command_char()

Remove superfluous function key_is_command_char()

The function key_is_command_char() is simply a predicate that determines
whether the function tui_dispatch_ctrl_char() will do anything useful.
Since tui_dispatch_ctrl_char() performs the same checks as
key_is_command_char() it is unnecessary to keep key_is_command_char()
around.  This patch removes this useless function and instead
unconditionally calls tui_dispatch_ctrl_char() inside its only caller,
tui_getc().

gdb/ChangeLog:

	* tui/tui-io.c (tui_getc): Don't call key_is_command_char.
	(key_is_command_char): Delete.