Remove check for zero focal length in BA cost functor

This check is actually redundant, because empty intrinsics
will have focal length of 1.0, which means original comment
about BundleIntrinsics was not truth.

It is possible that external user will send focal length of
zero to be refined, but blender prevents this from happening.
Cleanup in simple pipeline's bundler

- Better match Google's code style conventions.
- Move evaluation part into own function, makes
  bundling itself easier to follow.
- Made evaluation an optional parameter.
- Removed note about unsupported camera intrinsics
  refining flags. Technically, all combinations
  are possible.