luarocks: remove luainterpreter dependency

All packages using the luarocks infrastructure need a dependency on
luainterpreter, because having the Lua interpreter for the target is
needed to built native Lua modules. This dependency is already taken
care of in pkg-luarocks.mk.

However, host-luarocks, which is built as a dependency of the extract
step of any luarocks package, also had a dependency on
luainterpreter. Not only this was not necessary, but it was causing
problems with 'make legal-info'. Since 'make legal-info' triggers the
extraction of all packages, as soon as a luarocks package was enabled,
it would trigger the build of host-luarocks, itself triggering the
build of luainterpreter and therefore its dependencies, amongst which
the entire cross-compilation toolchain.

[Thomas: reword commit log to include more detailed explanations.]

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>