crypto: remove CRYPTO_TFM_RES_WEAK_KEY

The CRYPTO_TFM_RES_WEAK_KEY flag was apparently meant as a way to make
the ->setkey() functions provide more information about errors.

However, no one actually checks for this flag, which makes it pointless.
There are also no tests that verify that all algorithms actually set (or
don't set) it correctly.

This is also the last remaining CRYPTO_TFM_RES_* flag, which means that
it's the only thing still needing all the boilerplate code which
propagates these flags around from child => parent tfms.

And if someone ever needs to distinguish this error in the future (which
is somewhat unlikely, as it's been unneeded for a long time), it would
be much better to just define a new return value like -EKEYREJECTED.
That would be much simpler, less error-prone, and easier to test.

So just remove this flag.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>