Remove "cmd_type" function

The cmd_type function only has a single caller, which is in the CLI
implementation code.  This patch removes the function, and moves the
cmd_types enum definition from command.h to cli-decode.h, fixing an 18
year old FIXME.

gdb/ChangeLog
2020-06-28  Tom Tromey  <tom@tromey.com>

	* command.h (cmd_types): Remove.
	(cmd_type): Don't declare.
	* cli/cli-decode.h (enum cmd_types): Uncomment.  No longer a
	typedef.
	* cli/cli-cmds.c (setting_cmd): Use cmd->type directly.
	* cli/cli-decode.c (cmd_type): Remove.