[readme] Remove C++03 deprecation warning.

README.md

@@ -8,9 +8,6 @@ Boost.Geometry, part of collection of the [Boost C++ Libraries](http://github.co
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-| :warning: **CAUTION**: Boost.Geometry in Boost 1.73 deprecates support for the C++03 and will require C++14 from Boost 1.75 onwards (see issue [#590](https://github.com/boostorg/geometry/issues/590)). |
-| --- |
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