Revert "fu540 init: add a hack for broken rust riscv runtime"

Fixed by rust-lang/compiler-builtins#324

This reverts commit f506d39.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

Author: rminnich

src/soc/sifive/fu540/src/init.S

@@ -1,13 +1,3 @@
-// This is a GROSS HACK until rust is fixed to not expect
-// atomic load store on 16 bit quantities, which riscv
-// can not do.
-.globl __atomic_store_16
-__atomic_store_16:
-	ret
-.globl __atomic_load_16
-__atomic_load_16:
-	ret
-// END GROSS HACK
 
 .globl _stack_ptr
 .section .rodata