remove obsolete image handling description from README

Change-Id: I94023bc1a8eb1b6d31416292eccf220ba1c29efc
Reviewed-on: https://gerrit.libreoffice.org/67569Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>

svtools/README

 Tools on top of VCL. Common dialogs, file and print dialogs, wizards, vcl filters, lots of helper code.
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-== Image swapping ==
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-LO has two kind of swapout mechanisms:
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-1) Size based auto swapping: when the size of all graphic objects reaches a configurable limit (20 MB by default)
-then some of the graphics are saved to the local file system and freed in the memory.
-Configure: Tools -> Options -> Memory -> GraphicsCache -> Use for LibreOffice
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-2) Time based auto swapping: after an image is swapped in / loaded a timer starts and when the timer (1 min by default)
-has a time out this image is swapped out ( sometimes this time out is ignored when the image is in use ).
-Configure: Tools -> Options -> Memory -> GraphicsCache -> Remove from memory after
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-Both swapping are done by GraphicObject and GraphicManager.