Fonts: removed misleading SizePixels >= 0.0f test and stbtt_ScaleForMappingEmToPixels() call in ImGui_ImplStbTrueType_FontSrcInit(). (#8857)

Logic was picked in 9a9712807e while extracting code from stbtt_PackFontRangesGatherRects(), but ScaleForMappingEmToPixels() was actually never called: we assert against negative SizePixels since 2015.