Remove JBPF_EXPERIMENTAL_FEATURES from header files

[matthewbalkwill]

Due to the fact that JBPF_EXPERIMENTAL_FEATURE is used in Jbpf header files, and conditionally include fields in structures, all application code must also be built with this compile argument.

This is slightly messy.
Instead the JBPF_EXPERIMENTAL_FEATURE should be removed from header files and added to function bodies.