[libc++] Remove unnecessary std::vector accessors (#114423)

Now that we don't use __compressed_pair anymore inside std::vector, we
can remove some unnecessary accessors. This is a mechanical replacement
of the __alloc() and __end_cap() accessors, and similar for
std::vector<bool>.

Note that I consistently used this->__alloc_ instead of just __alloc_
since most of the code in <vector> uses that pattern to access members.
I don't think this is necessary anymore (and I'm even less certain I
like this), but I went for consistency with surrounding code. If we want
to change that, we can do a follow-up mechanical change.

Author: ldionne