Fix some resharper warnings

[nirinchev]

Description

This fixes a wide range of warnings/suggestions by ReSharper, primarily in the Realm project. Don't plan to go through the tests at this point, but figured I'd spend 20-30 minutes on the main assemblies. For the most part, those are purely cosmetic and functionally equivalent (e.g. removing usings, marking members private). There are a few categories of warnings that may yield a minor positive impact on users:

• Removed dead code
• Marked struct methods as readonly to avoid copying the struct prior to execution (this would have biggest impact on RealmValue and PrimitiveValue).
• Some ambiguous docs references.

TODO

• Changelog entry
• Tests (if applicable)

[coveralls]

Pull Request Test Coverage Report for Build 5750151873

• 50 of 58 (86.21%) changed or added relevant lines in 21 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.1%) to 82.943%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Realm/Realm/Helpers/SerializationHelper.cs	0	1	0.0%
Realm/Realm/Schema/ObjectSchema.cs	0	1	0.0%
Realm/Realm/Linq/RealmResultsVisitor.cs	8	10	80.0%
Realm/Realm/DatabaseTypes/RealmCollectionBase.cs	7	11	63.64%

Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Exceptions/RealmException.cs	2	78.95%
Realm/Realm/Handles/SessionHandle.cs	2	84.65%

Totals
Change from base Build 5743612663:	0.1%
Covered Lines:	6296
Relevant Lines:	7462

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💛 - Coveralls