[AArch64] Remove useless and incorrect assertion

[AArch64] Remove useless and incorrect assertion

The outer caller elf_link_output_extsym in elflink.c is a traverse function on
all external symbol, and it will only call *finish_dynamic_symbol if some
conditions is meet.  It is executed conditionally.

If the condition to trigger that assertion is satisified, it then won't satify
the outer check in finish_dynamic_symbol, so *finish_dynamic_symbol won't be
called that the assertion is expected to be dead code.

If elf_link_output_extsym is a traverse function that unconditionally called
on external symbols decided to be exported, then an assertion to make sure these
symbols are in sane status might make sense.

bfd/
        * elfnn-aarch64.c (elfNN_aarch64_finish_dynamic_symbol): Remove the
        sanity check at the head of this function.