Remove the -Csoft-float argument when building the kernel

The targets specify the soft-float ABI already and -Csoft-float should likely have never existed in the first place. It is unsound to mix crates compiled with different float ABI's and the only stable way to get all crates to agree on the soft float ABI is by specifying a target which uses the soft float ABI anyway even without explicit -Csoft-float.

See also https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.60-Csoft-float.60.20considered.20harmful

Depends on kernel!332 (merged)

According to https://github.com/rust-lang/rust/pull/36261, this flag anyway is a NOP for redox-os as none of the targets in https://gitlab.redox-os.org/redox-os/kernel/-/tree/master/targets?ref_type=heads are *eabihf targets.

mentioned in commit ea07606f

merged