Use gdbfmt for vprintf_filtered.

Use gdbfmt for vprintf_filtered.

gdbfmt was already used for printf_filtered, so using it for
vprintf_filtered is more consistent.

As a result, all callers of vfprintf_maybe_filtered now use gdbfmt, so
the function can be simplified to assume the gdbfmt case and remove
the associated bool argument.  Similary, vprintf_filtered is now a
simple wrapper around vfprintf_filtered.