Stop including tng_io_fwd.h
The two header files in GROMACS associated with TNG support only need
a forward declaration of the tng_trajectory type, and the typedef
tng_trajectory_t. For a build without TNG, that is all that is
required, because the pointer is never dereferenced. For a build that
uses TNG (whether bundled with GROMACS or installed on the systm), if
TNG no longer declares a type of that name, then compilation will fail
with or without this change.
Thus there is no change in quality of support of TNG from this change.
However, it does avoid a build configuration that does not use TNG
needing to be able to find one. This will be convenient for static
analysis support.
Also removed a useless include of tngio_for_tools.h.
Change-Id: I3438d3e1b0c38a62afec4ea3a7f6e977d6602d96