Remove use-package-font-lock-keywords

[belak]

I don't know how long use-package plans on supporting older versions of Emacs, or if it's alright to remove some minimal features supporting those versions (<=25), but it would be really nice to remove the font-lock keywords it adds. These are already highlighted on Emacs 26+ and shouldn't be needed on these versions. Additionally, this should not break anything other than font-lock on older versions of Emacs.

Doom removes it (https://github.com/hlissner/doom-emacs/blob/develop/core/core-modules.el#L343-L345) and I do as well to make use-package highlighted the same as other macros (I have a number of small wrappers which set some defaults and pass through all other forms for convenience).

[ronisbr]

Hi! I think this PR broke Doom Emacs:

https://github.com/doomemacs/doomemacs/issues/6699