SERVER-14123 Delete jstests/sharding/explain_large.js

explain_large.js spawns 18 shards and runs a query that allocates
300MB+ of memory on each.  This causes OOM conditions on MCI's 32-bit
buildvariants (limited to 4GB of addressable memory, no swap).

(cherry picked from commit 4dd1c9e70cc4a17d233c32180933da24b4c57ef5)
Merge remote-tracking branch 'upstream/v2.6' into v2.6