Remove gen_get_params & gen_gettable_params from keygen operation

Remove gen_get_params & gen_gettable_params from keygen operation

EVP_PKEY_CTX_gettable_params() was missing code for the keygen operation.
After adding it it was noticed that it is probably not required for this type, so instead
the gen_get_params and gen_gettable_params have been remnoved from the provider interface.
gen_get_params was only implemented for ec to get the curve name. This seems redundant
since normally you would set parameters into the keygen_init() and then generate a key.
Normally you would expect to extract data from the key - not the object that we just set up
to do the keygen.

Added a simple settable and gettable test into a test that does keygen.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11683)