Delete duplicate recording rules before adding unique index.

Commit af5836e9 added a unique index to the record table to prevent
the creation of duplicate recording rules, primarly from the internal
web interface or mythweb.  However, some users had already created
duplicate rules which caused the index creation to fail.  This commit
changes the update to delete the duplicate rules before creating the
index.
Merge branch 'master' of https://github.com/MythTV/mythtv into devel/webfrontend