Revert changes that were merged to master.

Reverted: 2e099caceb9..21102938e8ccb

I'm going to reapply some of them soon, but it's easier to revert them
all first, as there are conflicts. Also the number of changes that were
inserted since then, mean that there would be a huge gap between related
changes.
refs #6404

[SVN r80902]
refs #6403

[SVN r80903]
Add link to changeset mentioned in comment.
Merge branch 'feature/comment-fixes' into develop
Remove use of obsolete BOOST_TT_BROKEN_COMPILER_SPEC

[SVN r86250]
Remove all references to now defunct (and removed) header.

[SVN r86438]
Merge branch 'feature/remove-broken-compiler-spec' into develop