Description:
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Destroy and clear method do the same things but it is not specified in the 
documentation. I've seen that they use both functions in the Wordpress code, it is 
not so dramatic, but they are doing 2 functions call when only one would sufice.

One function should be the alias of the other to reduce code duplication too.

Test script:
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<?php

$imagick = new Imagick();
$imagick->clear();

$imagick = new Imagick();
$imagick->destroy();