Remove MERGE_ALT_FORM merge mode

Remove MERGE_ALT_FORM merge mode

The mode comes from the "alternate" keyword, which is unused in
xkeyboard-config and mostly undocumented. Its purpose is to allow to
assign the same key name to multiple key codes, which is not allowed
otherwise (and doesn't make much sense). The xkblib specification
implies that this was part of the overlay functionality, which we also
no longer support.

If we do encounter this keyword, we just treat it as MERGE_DEFAULT. The
keycodes.c code will detect a collision and will ignore all but the
first key code (and the error count is not incremented).

Some peripheral code is also removed as a result.

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