(PDB-904) Switch fact_values.value_string trgm index to be GIN not GIST

There were reports of crashing due to a bug in pg_trgm indexing when a large
fact value was loaded into PuppetDB.

This patch removes the old index via migration, and will then replace it with
the index handling mechanism.

Signed-off-by: Ken Barber <ken@bob.sh>