enhance: Remove commented code and fix naming issue

[congqixia]

This PR removes all the commented code and files from PR #28320

For naming issue:

• Renaming MinCheckpoint to EarliestPosition, see Use writebuffer, sync manager refactory in datanode #28320 comment
• Renaming writebuffer.Mananger to BufferMananger, see Add WriteBuffer to provided absbraction for delta policy #27874 comment

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: congqixia

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [congqixia]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[congqixia]

/kind improvement

[mergify]

@congqixia E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[congqixia]

/run-cpu-e2e

[codecov]

Codecov Report

Merging #28450 (0c7d802) into master (0b90507) will increase coverage by 0.04%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #28450      +/-   ##
==========================================
+ Coverage   81.71%   81.75%   +0.04%     
==========================================
  Files         842      842              
  Lines      117701   117702       +1     
==========================================
+ Hits        96176    96230      +54     
+ Misses      18289    18246      -43     
+ Partials     3236     3226      -10     

Files	Coverage Δ
internal/datanode/compactor.go	86.21% <100.00%> (ø)
internal/datanode/data_node.go	75.96% <ø> (ø)
internal/datanode/data_sync_service.go	76.19% <ø> (ø)
internal/datanode/flow_graph_time_tick_node.go	84.53% <100.00%> (ø)
internal/datanode/flow_graph_write_node.go	83.95% <ø> (ø)
internal/datanode/syncmgr/sync_manager.go	92.10% <100.00%> (ø)
internal/datanode/writebuffer/bf_write_buffer.go	85.00% <100.00%> (ø)
internal/datanode/writebuffer/l0_write_buffer.go	68.18% <100.00%> (ø)
internal/datanode/writebuffer/manager.go	84.15% <100.00%> (ø)
internal/datanode/writebuffer/segment_buffer.go	83.33% <100.00%> (ø)
... and 1 more

... and 15 files with indirect coverage changes

[mergify]

@congqixia E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[mergify]

@congqixia E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[mergify]

@congqixia E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[congqixia]

/run-cpu-e2e

[mergify]

@congqixia E2e jenkins job failed, comment /run-cpu-e2e can trigger the job again.

[congqixia]

/run-cpu-e2e

[XuanYang-cn]

/lgtm