Remove unneeded dependencies.

xmessage was used in the past by awsetbg https://github.com/awesomeWM/awesome/commit/ff713470169ff79d686499b20f53d72356730c23
imlib2 was replaced by gdk-pixbuf in 2012 https://github.com/awesomeWM/awesome/commit/c4a7f80e26306ee904b356f407fd1a6f47c6564d