rules: remove support for keymap rule

rules: remove support for keymap rule

This commit removes the ability to specify a keymap *in a rules file*,
e.g. in /usr/share/X11/xkb/rules/evdev or somesuch. This is unused in
xkeyboard-data, and the current code has never even supported it,
because xkb_map_new_from_kccgst (which is no longer exposed in the API)
checks to see that one of the usual components (e.g. symbols, types, ..)
has been filled, while the rules parser, on the other hand,  doesn't
allow to specify a keymap and other stuff at the same time.

( The idea was to remove xkb_map_new_from_kccgst entirely, but it's used
  by a test so it can stay. )

tl;dr: dead code. Of course passing a keymap file to
xkb_map_new_from_file still works.

Signed-off-by: Ran Benita <ran234@gmail.com>