toolchain: remove references to LIBFLOAT_TARGET

The libfloat package was removed in
accbd711, and therefore since this
commit, the LIBFLOAT_TARGET variable is always empty. We get rid of
the stale references to this variable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>