support for nsPrefBranch::{get,set}StringPref() (#901)

support for nsPrefBranch::{get,set}StringPref() (#901)

Firefox commit:

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Bug 1414096 (attempt 2) - Remove support for nsISupportsString values in nsPrefBranch::{get,set}ComplexValue(). r=florian.

Bug 1345294 introduced nsPrefBranch::{get,set}StringPref(), which allowed the
getting of utf8 strings from prefs, which previously required using
nsISupportsString with {get,set}ComplexValue. That bug also converted most
uses.

This patch finishes the job.

- It removes the nsISupportsString support.
- It converts existing code that relied on the nsISupportsString.
- It removes the lint that was set up to detect such uses of nsISupportsString.

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