[entropy_src] Remove stale configuration
[entropy_src] Remove stale configuration

- an lfsr mode is no longer configurable for entropy_src, therefore
  the related seed registers no longer exist.

- remove the usage of seed registers as part of the parameterized
  stub mode.

- fixes #14427

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan