Remove the "search_documentfield" table

Summary: Ref T11741. See PHI1276. After the switch to "Ferret", this table has no remaining readers or writers.

Test Plan:

• Ran bin/storage upgrade -f, no warnings.
• Grepped for class name, table name, stemmedCorpus column; got no relevant hits.
• Did a fulltext search.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11741

Differential Revision: https://secure.phabricator.com/D20549