[csrng,doc] Remove second definition of EDN acronym
[csrng,doc] Remove second definition of EDN acronym

This acronym has been already defined just a few sentences before.

Signed-off-by: Andreas Kurth <62c4a7f75976b7a2fb8d79afddaccc3e9236aad5@lowrisc.org>

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