fix(account):if new account can't pay NewAccountTax, it must be removed

When an account was deleted, it was not really deleted, its balance was just set to zero.

So the account was still considered as existing from the point of view of the System::account_exists() function.

Closes #60 (closed)

changed milestone to %runtime-103

requested review from @tuxmain

approved this merge request

changed target branch from master to release/runtime-100

merged