toplevel: do not make libstdc++ configure wait for libgomp

libstdc++'s configure was made to depend on libgomp's configure by
r0-83445-g6d3b5aea0bf4c9 ("configure.ac (TOPLEVEL_CONFIGURE_ARGUMENTS): Move
libgomp before libstdc++.", 2007-09-20), when GLIBCXX_ENABLE_PARALLEL decided
whether to enable parallel mode by looking for a configured libgomp/omp.h on
disk:

  if test -f $glibcxx_builddir/../libgomp/omp.h; then

r0-98141-gaae29963cf81dd ("PR libstdc++/36101, PR libstdc++/42813",
2010-01-22) replaced that with

  if echo " ${TARGET_CONFIGDIRS} " | grep " libgomp " > /dev/null 2>&1 ; then

so the test now reads a string the top level passes in, and nothing has to
exist on disk by then.  The comment above it still describes the old
behaviour, which is what makes the edge look necessary.

Remove it.  The build time dependency, all-target-libstdc++-v3 on
configure-target-libgomp, stays, since the parallel mode headers do include
omp.h when they are compiled.

This shortens the chain in front of libstdc++, which is the last thing to
finish in every bootstrap stage.

Bootstrapped on aarch64-none-linux-gnu.

ChangeLog:

	* Makefile.def (dependencies): Remove the configure-target-libstdc++-v3
	on configure-target-libgomp dependency.
	* Makefile.in: Regenerate.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>