Drop old Audit tables; make markup cache binary

Summary:
Ref T1191.

• Fixes T6096. We've migrated away from this table in T4896. The data is now in the transaction table. There have been no reads or writes to this table for some time and I haven't seen any issues from users.
• Fixes T6097. Same deal as above. The data is now in the transaction comment table.
• Fixes T6100. This cache is safe to wipe out, since it's purely read-through. Wiping it will make the migration faster. The column type change fixes storage of PHP serialized objects in a text column.

Test Plan:

• Ran migrations.
• Observed some yellow go blue on the Database Status screen.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6096, T6100, T6097, T1191

Differential Revision: https://secure.phabricator.com/D10520