Remove redundant .cargo/config

All options are already set by ../recipe.toml, and removing this file
will make Cargo stop complaining about missing -Zbuild-std when for
example compiling RMM for the host arch.

.cargo/config

@@ -1,8 +0,0 @@
-[build]
-rustflags = [
-  # Kernel should preserve floating-point registers
-  "-Csoft-float",
-]
-
-[unstable]
-build-std = ["core", "alloc", "compiler_builtins"]