remove ignored restrct return type qualifier

The use of restrict in the return type qualifier for mca_btl_vader_reserve_fbox
is being ignored by gnu compiler.  for newer gcc, one sees this warning only
with -Wignored-qualifiers set, but for older variants of gcc it was reported
that numerous warning messages about this ignored qualifier were being
generated as vader is being compiled.

The warning reported by gcc is

btl_vader_fbox.h:53:47: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 static inline mca_btl_vader_fbox_t * restrict mca_btl_vader_reserve_fbox (struct mca_btl_base_endpoint_t *ep, const size_t size)
Revert r32675 in favor of a different solution proposed by Brice