Remove soft-float flag

`-Csoft-float` is deprecated. Soft float is enabled by the target spec.

    $ rustc -Z unstable-options --target=x86_64-unknown-uefi --print target-spec-json | grep features
    "features": "-mmx,-sse,+soft-float",

Ref: rust-lang/rust#129893
Signed-off-by: Tim Crawford <[email protected]>

Makefile

@@ -51,5 +51,4 @@ $(BUILD)/boot.efi: Cargo.lock Cargo.toml src/* src/*/*
 		--target $(TARGET) \
 		--release \
 		-- \
-		-C soft-float \
 		--emit link=$@