[libc++][format] Removes vector dependency.

During the review of D140653 it was suggested to use vector in
__retarget_buffer instead of manually managing the memory. Due to the
requirements of the Standard it turns out format needs to include vector
leading to a cycle. Therefore switching back to manual memory
management.

This is a preparation to fix https://llvm.org/PR61314

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D148826