gdb/testsuite: remove use of sleep from gdb.base/foll-vfork.exp

gdb/testsuite: remove use of sleep from gdb.base/foll-vfork.exp

While working on gdb.base/foll-vfork.exp I noticed that there are
several random 'sleep' calls throughout the test.

The comment suggests these are to allow for output from a vforked
child to arrive, but in each case the test is about to close and
restart GDB, so I don't see how random output from a child process
could impact testing.

I removed the sleep calls and couldn't reproduce any failures from
this test, I left the test running for a couple of hours, and tried
loading my machine, and the test seems fine with these removed.

I've left this as a separate commit so that if, in the future, someone
can show that these are required, it will be easy to revert this one
patch and bring them back.

There should be no change in what is tested after this commit.