Removed @Beta for toImmutableList and toImmutableMap

[yuyumao]

#3251

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[yuyumao]

I signed it!

[googlebot]

CLAs look good, thanks!

[zshamrock]

@yuyumao just to double check do you also consider to remove @Beta from other immutable collection classes, like

• ImmutableBiMap,
• ImmutableListMultimap,
• ImmutableSet,
• ImmutableSetMultimap,
• ImmutableSortedMap,
• ImmutableSortedSet

Maybe there are others, but these are the ones I've found.

[yuyumao]

@zshamrock I've added all the classes you listed

[danielhultqvist]

@netdpb Any updates on this PR? Seems that the failing Travis build is probably unrelated. My team are hesitant of using the collectors as it warns about the usage in IDEA

[cpovirk]

#3393