With https://bugzilla.mozilla.org/show_bug.cgi?id=844349 now being fixed in Firefox, the textLayer will now actually stay hidden as intended regardless of the browser settings.
Hence it should no longer be necessary to display the fallback bar, nor print a warning in the console, for documents which contains a textLayer.
Besides removing the `supportsDocumentColors` methods in the default viewer, we can also remove a now unused l10n string.
Most of the files in this folder (except for the en-US folder) have been
imported from the Firefox Nightly branch;
please see https://hg.mozilla.org/l10n-central. Some of the files are
licensed under the MPL license. You can obtain a copy of the license at
https://mozilla.org/MPL/2.0.
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