Revert "Some SvxFontHeightItem clean-up"

This reverts commit d5f93eb47ec4c27e93ad908b0199790c702491a0,
SvxFontHeightItem::SetProp was used to override "dummy" 100% nProp/ePropUnit
values that did not match the computed nHeight value with nProp/ePropUnit values
that are supposed to match the computed nHeight value.  Yuck.

Change-Id: Ic12e824054b3c5da7aca766b9b3eb076d1837d9a
Fix SvxFontHeightItem::operator=

...and remove thus unused SetHeightValue

Change-Id: I9c8c18614233df6086ff23533e7e7129848f42ef
Copy the original prop/prop unit

...instead of scaling the translated nNewHeight once more by the original prop

Change-Id: Iadb53361a79f0b37b3c7c50066e4cfb7d24ed5e8