[lldb] Delete Value::Vector class

This class and it's surroundings contain a lot of shady code, but as far
as I can tell all of that code is unreachable (there is no code actually
setting the value to eValueTypeVector).

According to history this class was introduced in 2012 in
r167033/0665a0f0. At that time, the code seemed to serve some purpose,
and it had two entry points (in Value::SetContext and
ClangExpressionDeclMap::LookupDecl). The first entry point was deleted
in D17897 and the second one in r179842/44342735.

The stated purpose of the patch introducing this class was to fix
TestRegisters.py, and "expr $xmm0" in particular. Both of these things
function perfectly well these days without this class.