msetup: remove bad warning about unused options

This is just a bad warning, while it *could* give the user useful
information, it often doesn't since it can get values to warn about
from:

  - environment variables
  - the command line
  - machine files
  - `project(default_options : ...)`
  - `subproject(default_options : ...)`
  - `dependency(default_options : ...)`

The problem of course is that user may have no control over these
values. 3 of them are hardcoded into the meson.build files, so the user
can't do anything about them. And there are legitimate reasons to have
unused values in those, like setting defaults for a language only used
on specific platforms.

Environment variables may be set by the distro (NixOS sets them for any
enabled language, so just having a D compiler causes `DFLAGS` to be set,
for example). They likely don't want to special case "only set the
environment variables if the project is going to use them".

For machine files it limits the utility of the files, since the user
needs to be sure that they don't include any options that wont be used.

Finally, the command line could be altered by wrapper scripts, or simply
programmed to insert options that *may* be used but aren't required.
like setting `objc_args` regardless of whether ObjectivC bindings are
generated.

However, passing completely unknown builtin options should be an error,
as it was before the optionrefactor

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