docs: Remove old style HACKING

The information is outdated, the relevant documentation is all in
[the README](data/theme/README) nowadays.

data/theme/HACKING

@@ -1,3 +0,0 @@
-To generate the css files, from the project directory:
-
-sass --sourcemap=none --update .

data/theme/gnome-shell-sass/HACKING

@@ -1,6 +0,0 @@
---- Generating the css file ---
-
-You need sass to generate the css file.
-
-To generate them run from a command line in the project directory:
-sass --sourcemap=none --update ./