riscv: remove redundant mv instructions

Some mv instructions were useful when first introduced to preserve a0 and
a1 before function calls. However the code has changed and they are now
redundant. Remove them.

Signed-off-by: Nam Cao <namcaov@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230725053835.138910-1-namcaov@gmail.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>