Increase static linkage, remove unused functions

A large number of functions were defined with external linkage, even
though they were only used from within one file. This patch changes
their linkage to static and removes the vp8_ prefix from their names,
which should make it more obvious to the reader that the function is
contained within the current translation unit. Functions that were
not referenced were removed.

These symbols were identified by:

  $ nm -A libvpx.a | sort -k3 | uniq -c -f2 | grep ' [A-Z] ' \
    | sort | grep '^ *1 '

Change-Id: I59609f58ab65312012c047036ae1e0634f795779