Update uglify-overview.md

Remove upgrade instructions that never applied since v3.x is a version from the future.

docs/uglify-overview.md

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 Task targets, files and options may be specified according to the grunt [Configuring tasks](http://gruntjs.com/configuring-tasks) guide.
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-# Migrating from 2.x to 3.x
-
-Version `3.x` introduced changes to configuring source maps. Accordingly, if you don't use the source map options you should be able to upgrade seamlessly. If you do use source maps, see below.
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-## Removed options
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-* `sourceMappingURL` - This is calculated automatically now
-* `sourceMapPrefix` - No longer necessary for the above reason
-
-## Changed options
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-* `sourceMap` - Only accepts a `Boolean` value. Generates a map with a default name for you
-* `sourceMapRoot` - The location of your sources is now calculated for you when `sourceMap` is set to `true` but you can set manual source root if needed
-
-## New options
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-* `sourceMapName` - Accepts a string or function to change the location or name of your map
-* `sourceMapIncludeSources` - Embed the content of your source files directly into the map
-* `expression` - Accepts a `Boolean` value. Parse a single expression (JSON or single functions)
-* `quoteStyle` - Accepts integers `0` (default), `1`, `2`, `3`. Enforce or preserve quotation mark style.