riscv, bpf: Remove duplicated bpf_flush_icache()

The bpf_flush_icache() is done by bpf_arch_text_copy() already.
Remove the duplicated one in arch_prepare_bpf_trampoline().

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/bpf/20250904105119.21861-1-hengqi.chen@gmail.com