Remove ImagePalette._make_gamma_lut?

`ImagePalette._make_gamma_lut()` is an internal factory class but isn't used anywhere in `PIL/*` or `Tests/*` code.

It's been in Pillow since the PIL fork, and was added in PIL 1.1.7. The `_make_linear_lut()` function was added at the same time and is still in use. Here's the relevant diff:
http://hg.effbot.org/pil-2009-raclette/commits/915c68a71749325ce110b697d2740c0977afea55

Shall we remove `_make_gamma_lut()`?

(Two commits in this PR: one to remove it, and one for flake8.)