Clean up the QSO code for the Main Survey

[geordie666]

This PR removes older cuts for selecting QSOs, to simplify the code prior to processing the on-sky Main Survey targets. It removes:

• Any Main Survey code that was used to select QSOs for data releases prior to DR9 of the Legacy Surveys.
• The Main Survey code that selects high-redshift quasars using the Random Forest (the QSO_HIZ quasars).
  • The QSO_HIZ bit is retained, it's just never set.

In addition, the code fixes a bug where the ELG_VLO and ELG_LOP Main Survey target classes were being assigned the same priorities. Now, the initial priorities for these ELG classes are:

• 3000 for ELG_VLO.
• 3100 for ELG_LOP.

[coveralls]

Coverage increased (+0.1%) to 59.393% when pulling 311112f74a2de2f083a8a42764567a8be8004db3 on ADM-QSO-code-clean into e073dd303fee60657f015dcc57955143d5413429 on master.