Remove user.provisional column.

This is leftover from a previous iteration of ATAT where inviting a user
to a portfolio would create a pending entry in the users table. This is
no longer used.

atst/models/user.py

@@ -1,4 +1,4 @@
-from sqlalchemy import String, ForeignKey, Column, Date, Boolean, Table, TIMESTAMP
+from sqlalchemy import String, ForeignKey, Column, Date, Table, TIMESTAMP
 from sqlalchemy.orm import relationship
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.event import listen
@@ -67,8 +67,6 @@ class User(
     last_login = Column(TIMESTAMP(timezone=True), nullable=True)
     last_session_id = Column(UUID(as_uuid=True), nullable=True)
 
-    provisional = Column(Boolean)
-
     cloud_id = Column(String)
 
     REQUIRED_FIELDS = [