`TablesNextToCode` is now a substituted by configure, where it has the
correct defaults and error handling. Nowhere else needs to duplicate
that, though we may want the compiler to to guard against bogus settings
files.

I renamed it from `GhcEnableTablesNextToCode` to `TablesNextToCode` to:

 - Help me guard against any unfixed usages

 - Remove any lingering connotation that this flag needs to be combined
   with `GhcUnreigsterised`.

Original reviewers:

Original subscribers: TerrorJack, rwbarton, carter

Original Differential Revision: https://phabricator.haskell.org/D5082