lib: add vector_remove() to vector.[ch]

lib: add vector_remove() to vector.[ch]

An optimized version of this has already been implemented within graph.c
that assumes some specialized constraints for that code. It's generally
useful so this change implements a general purpose version of it.

This fixes cmd_make_strvec() that was broken by some code shuffling in
previous commits.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>