modpost: remove stale comment about sym_add_exported()

The description,

  it may have already been added without a
  CRC, in this case just update the CRC

... is no longer valid.

In the old days, this function was used to update the CRC as well.

Commit 040fcc81 ("kbuild: improved modversioning support for
external modules") started to use a separate function (sym_update_crc)
for updating the CRC.

The first part, "Add an exported symbol" is correct, but it is too
obvious from the function name. Drop this comment entirely.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>