MINOR: receiver: Remove tgroup_mask from struct shard_info

MINOR: receiver: Remove tgroup_mask from struct shard_info

The only purpose from tgroup_mask seems to be to calculate how many
tgroups share the same shard, but this is an information we can
calculate differently, we just have to increment the number when a new
receiver is added to the shard, and decrement it when one is detached
from the shard. Removing thread group masks will allow us to increase
the maximum number of thread groups past 64.