Remove support for waiting for executors in standalone mode.

Current code waits until some minimum fraction of expected executors
have registered before beginning scheduling.  The current code in
standalone mode suffers from a race condition (SPARK-2635). This
race condition could be fixed, but this functionality is easily
achieved by the user (they can use the storage status to determine
how many executors are up, as described by @pwendell in #1462)
so adding the extra complexity to the scheduler code may not be worthwhile.

This commit removes the functionality in standalone mode but not for
YARN -- where it is more necessary and the number of expected executors
is well-defined.

This PR is a POC; if the powers-that-be determine that this is what we should
do, I will file a JIRA.

This should be backported into 1.1 if committed.