First part of fixes related to governance vote expiration

[Fix] removed erroneous addition to clear_account routine - it no longer resets proxy and recovery account or expire governance vote

[Fix] in AH account_witness_proxy_operation no longer binds to no-account when it is clearing proxy

[Cleanup] proxy member of account_object isolated - all operations on it have to go through member functions - it should enable us to turn it into id instead of string (that would impact order of one version of database_api::list_accounts so it was not changed yet). The main reason for the change was to force all comparisons to be made with HIVE_PROXY_TO_SELF_ACCOUNT - previously some places in code assumed that its value is empty string.

[Cleanup] HF23 and HF24 no longer have activation times controlled through env.var. - such flexibility is only ever needed for incoming HF