ext: lib: tinycrypt: do not select entropy_generator

Tinycrypt ECC_DH and ECC_DSA are selecting the entropy generator, this
is not required. ECC_DH and ECC_DSA require a default_CSPRNG definition
that could be based on the entropy generator, but the entropy generator
does not provide this function.

Removing the entropy generator results in smaller code in case only
validation needs to be done (which does not need default_CSPRNG). The
modules that are providing the default_CSPRNG (e.g. subsys/jwt/jwt.c or
subsys/bluetooth/host/hci_ecc.c) are deciding themselves to use the
entropy generator or not.

Signed-off-by: Laczen JMS <laczenjms@gmail.com>