libsnapshot: remove op count check

With variable block size compression being added, the number of ops
written cannot be calculated directly as easily since one op can cover
the data for multiple ops previously. We can get rid of this check for
XOR and Raw blocks as
within WriteOperation() we already make a check to see if we are
exceeding op_count_max limit.

We still need to keep this check for EmitZeroBlocks and EmitCopyBlocks
since the number of operations is determined ahead of time in those
function calls. Without this check in place, the ops will be added to
cached ops and return true when ops cannot be written.
with this change, v3 cow ota now works on cuttlefish with support for
variable block size compression.

Test: th
Change-Id: Ia55f152f5deb67a9022d0feff112345e72741dd3