Remove code that tweaks rasterized glyphs in QFontEngineS60::alphaMapForGlyph()

Symbian's CFont::GetCharacterData() returns strange rasterized Glyph 8-bit data. The data values are 0x00, 0x10 ... 0xe0, 0xf0. So, a real opaque 0xff is never reached. Non-opaqueness is bad (performance wise) if we blit such glyphs in the raster paint engine.

Therefore, some code was added to QFontEngineS60::alphaMapForGlyph() which changes all 0xf0 to 0xff.

However, rumors say that this bug has been fixed in Symbian (more exactly in the font rasterization plugin). So, maybe this is not required for Symbian^3 (which uses OpenVG instead of raster, anyways).

Also, the raster paint engine does not use QFontEngineS60::alphaMapForGlyph(), since quite some time ...after that code was added to QFontEngineS60::alphaMapForGlyph().