firmware: Remove handling of R_RISCV_{32,64}

Since everything is statically linked, we won't actually have
R_RISCV_{32,64} relocations. No need to handle these.

Fixes: 0f20e8ad

 ("firmware: Support position independent execution")
Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Anup Patel <anup@brainfault.org>