Link the encoder core directly to the VP library

This avoids using the codepaths for dynamically loading the
DLL, letting the linker resolve it as any other function.
Remove everything related to NO_DYNAMIC_VP

This mode is the default and used everywhere now.
Remove a TODO that is no longer relevant

The function signatures are equal nowadays.
Remove the now unused intermediate code for loading the VP library dynamically
Merge pull request #647 from mstorsjo/remove-dynamic-vp

Remove code for loading the VP library dynamically