Remove unneeded variable usage analysis.

Remove unneeded variable usage analysis.

A variable usage analysis pass was run on toplevel and lazily-compiled
code but never used.  Remove this pass and the data structures it
builds.

The representation of variable usage for Variables has been changed
from a struct containing a (weighted) count of reads and writes to a
simple flag.  VariableProxies are always used, as before.  The unused
"object uses" is removed.

Review URL: http://codereview.chromium.org/669270

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4052 ce2b1a6d-e550-0410-aec6-3dcde31c8c00