Added weakValues() to MultimapBuilder.java

[Ayush-Thakur-geek]

Fixes for issue id #6985

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[netdpb]

Hi @Ayush-Thakur-geek. Thanks for your PR. A few high-level comments:

1. We haven't yet decided to support weak values in Multimaps at all (Support for weak values in Multimap #6985), so a PR is premature at this point.
2. Your PR appears to add a method that has no effect. The request is to add support for weak values, not simply a method called weakValues.

As I said, we would have to decide that the feature is worth the trouble to design and implement before we'd accept implementations anyway. I'll close this PR for now.