Replace Vector _grow/_shrink checks with assert.

On review of the code, _grow and _shrink are checking for conditions
that cannot happen and that don't even really make sense.  For
example, if _shrink is called with where + amount > mCount then
this is really bad, however the check only considered the case
when where >= mCount and then it would arbitrarily choose a new
value for where.  Huh?

As it happens, the callers are correctly validating the
arguments before passing them down to these methods so we can
get rid of this code.

Change-Id: I921852dba8997065bb0e9cac733e82028d14afcd