Remove redundant RFFPWAIT/vfork(2) handling in Linux fork(2) and clone(2) wrappers.

r161611 added some of the code from sys_vfork() directly into the Linux
module wrappers since they use RFSTOPPED. In r232240, the RFFPWAIT handling
was moved to syscallret(), thus this code in the Linux module is no longer
needed as it will be called later.

This also allows the Linux wrappers to benefit from the fix in r275616 for
threads not getting suspended if their vforked child is stopped while they
wait on them.

Reviewed by: jhb, kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D3828