First round of removing memoize

https://jira.gsc.wustl.edu/browse/AT-650

This first round of fixes removes all memoizes in VariantReporting that were used on an object instance subroutine without any additional parameters. They were converted into transient attributes.
If every place that needs the master header is going to immediately dereference it, would it be better to `return @{$self->_master_header};` here and leave the other code unchanged?  (This has the additional benefit of not letting other code accidentally mess up the cache by manipulating the reference.)