bitbake.conf: Drop lz4 from HOSTTOOLS

bitbake.conf: Drop lz4 from HOSTTOOLS

As far as I can tell, we don't have anything using lz4 in our key
build dependencies. It isn't in ASSUME_PROVIDED and our code automatically
adds lz4-native dependencies where needed. Even then, it is very rarely
needed, mainly for some kinds of SRC_URI (none in OE-Core) and some
filesystems/images.

As such, drop this from HOSTTOOLS and rely on lz4-native, which is what
was already happening anyway. This simplies host setup slightly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>