Remove unused code

Summary:
Based on D30451379, applied on the latest version of `erl_scan.erl`.
Also rename the module so it does not override the OTP one during CI.
The module will be renamed into `elp_scan.erl` when ready.

Reviewed By: alanz

Differential Revision: D57724510

fbshipit-source-id: 533e0c8ba5ad8403371a9155da54de917ece871f