[m-rev.] for review: make --warn-unused-imports the default

[Zoltan Somogyi]

The diff is trivial; I am asking for opinions on the design decision.

In some rare cases, an import is needed only in some circumstances
(such as grades), and the compiler generates a unused import warning
for it in other circumstances. If this happens, we can always work around
the problem by adding --no-warn-unused-imports to the module's
entry in Mercury.options while (if possible) teaching the compiler
not to generate spurious warnings in such circumstances.

Zoltan.
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