Remove some unnecessary declarations

- topexcl.h was an outdated copy of gpp_nextnb.h, and wasn't used
  anywhere.  Rename the source file to gpp_nextnb.c to match the header.
- Remove one declaration from gpp_nextnb.h, which wasn't used outside
  the source file.  Make it static in the source file.
- Remove duplicate declarations of (init|done)_top().
- Remove unused create_visibility() from src/programs/view/.

Change-Id: I87982e5581aff8caca3dc753492e01ee457ab694