The dbtest binary should not be built as part of the compile phase

We include the dbtest binary among the targets in the compile task:

https://github.com/mongodb/mongo/blob/e50a4cb38a22e95c33bc1e803bb05be11e8643cb/etc/evergreen.yml#L3338

There is no point to doing so, because we re-request its construction in the dbtest! group:

https://github.com/mongodb/mongo/blob/e50a4cb38a22e95c33bc1e803bb05be11e8643cb/etc/evergreen.yml#L3574

As a result, the compile phase of the dbtest! group is usually a no-op build:

https://evergreen.mongodb.com/task_log_raw/mongodb_mongo_master_enterprise_rhel_62_64_bit_compile_dbtest_bf895a8f816b9bae68eb22f349b45e1d1d5f2da7_19_03_05_16_52_11/0?type=T#L155

We should simply remove dbtest from the targets in the compile phase.