Simplify write_inferior_memory

Simplify write_inferior_memory

gdbserver's write_inferior_memory uses a static variable to avoid
memory leaks, and has a comment referring to the lack of cleanups.
This patch removes this comment and the code in favor of a
straightforward use of std::vector.

gdb/gdbserver/ChangeLog
2019-08-15  Tom Tromey  <tromey@adacore.com>

	* target.c (write_inferior_memory): Use std::vector.