ex_cmds.lua: Use ex_ni directly

As can be seen in [1], the ifdefs are leftovers from Vim, where tons of
features are optional.  This is not the case in Nvim, so the macros are
of little use.  Besides, ex_ni is already used in a bunch of places,
just not consistently.

The ex_language macro was left untouched as the inclusion of
``:language'' actually is determined at compile time, unlike the other
commands which _always_ print "this command is not available in this
version".

[1]: https://github.com/vim/vim/blob/master/src/ex_docmd.c#L18-L497

Reviewed-by: Justin M. Keyes <justinkz@gmail.com>

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