Drop old/obsolete symfile
This would, at least on some setups, result in actual API symbols missing from the library, because the symfile hasn't been changed for over a decade.
Additionally, it should not be needed anymore as the project appears to use -fvisibility=hidden
and actually tag the public API symbols appropriately.
This fixes tests build with LTO (as long as static libs are not disabled) besides other issues.
ref #17