backup: remove duplicate kv size summary collect (#471)

[ti-srebot]

cherry-pick #471 to release-4.0

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What problem does this PR solve?

resolve #470

What is changed and how it works?

we calculate duplicate in backup summary log. the first time is in BackupRange, and the second time is in checksum, we should remove the second one because not every backup task starts with checksum.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  before this PR in v4.0.4:

[2020/08/25 10:36:22.567 +08:00] [INFO] [collector.go:60] ["Table backup Success summary: total backup ranges: 1, total success: 1, total failed: 0, total take(s): 0.13, total kv: 71996, total size(MB): 72.23, avg speed(MB/s): 547.18"] ["backup checksum"=127.091977ms] ["backup fast checksum"=217.299µs] ["backup total regions"=1] [BackupTS=418990779862614018] [Size=530514]

after this fix:

[2020/08/25 10:33:04.525 +08:00] [INFO] [collector.go:60] ["Table backup Success summary: total backup ranges: 1, total success: 1, total failed: 0, total take(s): 0.21, total kv: 35998, total size(MB): 36.12, avg speed(MB/s): 173.17"] ["backup checksum"=39.668497ms] ["backup fast checksum"=281.476µs] ["backup total regions"=1] [BackupTS=418990727944732675] [Size=530629]

Related changes

• Need to cherry-pick to the release branch

Release Note

• Fix the issue that total kv and total bytes calculate duplicate in backup summary log.

[kennytm]

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/2706/pipeline

[2020-08-25T03:27:25.655Z] *===== Running test tests/br_tiflash/run.sh... =====*
[2020-08-25T03:27:25.655Z] ERROR 1105 (HY000) at line 1: the tiflash replica count: 1 should be less than the total tiflash server count: 0

c'mon we're in the special week

[kennytm]

/run-integration-tests