Revert "error: make the module depend on vfprintf-posix"

Revert "error: make the module depend on vfprintf-posix"

This reverts commit 88e5b4d4641f294d8f824d770c9c55a1abe49768.

The GPL 'error' module is used by some of the gnulib testsuites;
and for some projects that use LGPL 'vasnprintf' in their lib/
directory, having 'error' depend on GPL 'vfprintf-posix' meant
that compilation breaks for those projects:

autoreconf: running: aclocal -I m4 --force -I m4
missing file gnulib/tests/vasnprintf.c
configure.ac:135: error: expected source file, required through
AC_LIBSOURCES, not found
m4/gnulib-comp.m4:518: gl_INIT is expanded from...
configure.ac:135: the top level
autom4te: /usr/bin/m4 failed with exit status: 1

I suspect that the problem encountered on OS/x was a warning from
gcc complaining about inconsistent use of __attribute__((__format__
__printf__)) vs. ((__format__ __gnu_printf__)); so perhaps the
real fix is to teach error.h to copy the extra care taken by
stdio.h with regards to whether merely system or full GNU printf
specifiers are being handled.  But without actually reproducing
that situation myself, the easiest action for now is to just revert
the dependency that violates the 'Comment' section of vfprintf-posix.