tjLoadImage(): Don't convert RGB to grayscale

tjLoadImage(): Don't convert RGB to grayscale

Loading RGB image files into a grayscale buffer isn't a particularly
useful feature, given that libjpeg-turbo can perform this conversion
much more optimally (with SIMD acceleration on some platforms) during
the compression process.  Also, the RGB2GRAY() macro was not producing
deterministic cross-platform results because of variations in the
round-off behavior of various floating point implementations, so
`tjunittest -bmp` was failing in i386 builds.