feat: Remove spark connector as a module

[jiangmichaellll]

Since it's moved into java-pubsublite-spark and no longer maintained here.

fixes #450

[anguillanneuf]

Thanks for this. The source code now lives in https://github.com/googleapis/java-pubsublite-spark

[codecov]

Codecov Report

Merging #467 (5e38d80) into master (8b00e43) will increase coverage by 1.54%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #467      +/-   ##
============================================
+ Coverage     71.55%   73.09%   +1.54%     
+ Complexity      913      835      -78     
============================================
  Files           168      151      -17     
  Lines          4859     4368     -491     
  Branches        209      222      +13     
============================================
- Hits           3477     3193     -284     
+ Misses         1263     1056     -207     
  Partials        119      119              

Impacted Files	Coverage Δ	Complexity Δ
...oud/pubsublite/internal/wire/SingleConnection.java	74.35% <0.00%> (-7.34%)	16.00% <0.00%> (ø%)
...cloud/pubsublite/internal/wire/SubscriberImpl.java	83.33% <0.00%> (-2.04%)	27.00% <0.00%> (ø%)
...bsublite/internal/wire/RetryingConnectionImpl.java	65.00% <0.00%> (-2.02%)	12.00% <0.00%> (ø%)
...lite/cloudpubsub/internal/AssigningSubscriber.java	86.00% <0.00%> (-1.76%)	11.00% <0.00%> (ø%)
...sublite/internal/wire/ConnectedSubscriberImpl.java	91.93% <0.00%> (-1.51%)	14.00% <0.00%> (ø%)
...internal/wire/PartitionCountWatchingPublisher.java	97.08% <0.00%> (-0.96%)	18.00% <0.00%> (ø%)
...m/google/cloud/pubsublite/beam/PublisherCache.java	45.45% <0.00%> (ø)	3.00% <0.00%> (ø%)
...oud/pubsublite/beam/PubsubLiteUnboundedReader.java	75.42% <0.00%> (ø)	24.00% <0.00%> (ø%)
.../cloud/pubsublite/internal/wire/PublisherImpl.java	89.38% <0.00%> (ø)	24.00% <0.00%> (ø%)
...oogle/cloud/pubsublite/v1/CursorServiceClient.java	82.00% <0.00%> (+2.00%)	14.00% <0.00%> (ø%)
... and 1 more

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