dev-util/ccache: drop misleading install message

dev-util/ccache: drop misleading install message

Even being relatively long readme.doc has a few limitations:
- non-standard ccache usage requires a bit more fiddling than readme suggests
- standard ccache usage is not covered fully (not make.conf tweaks, no caveats)

Let's leave all the details to https://wiki.gentoo.org/wiki/Ccache page.

Reported-by: Esteve Varela Colominas
Closes: https://bugs.gentoo.org/712256
Package-Manager: Portage-2.3.93, Repoman-2.3.20
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>